Blowback 9mm Carbines

[BreechFace]

There has been a huge market for PCC's in 9mm; and a lot of folks are choosing to assemble their own from parts. I would say 90%+ of the PCC's in 9mm are blowback operated. There are some definite items that need to be understood to do so safely.

I was just reading on @MySuomi's thread on his JRC OOB detonation and it just spurned a thought that we should get some information out there for what works on blowback 9mm carbines for those who wish to assemble one, or diagnose problems with one.

In my own experience putting together some PCC's I've always used the following information and have had good experiences:

How much reciprocating mass do we need for 9mm blowback?

Total weight for BCG and buffer should equal 21-24 oz. Loads that one develops for the PCC should be considered (i.e. standard P, +P, +P+, etc) when determining which side of the scale of oz. above one is at.

Generally, most 9mm bolts are 15 oz. so adding a 6.5-9 oz. buffer would suffice (if removing weight from the carrier additional buffer weight will be needed to make up the difference, see below).

If combining with a Law Tactical Folder or Sylvan Arms Folding Adapter the folding adapter extension which fits into the rear of the BCG’s weight should be considered in the total calculation, they typically weight ~2 oz. (see note below)

NOTE: there are a lot of PCC BCG's that have weight added to the tail of the carrier, some are removable and some are not. The removable weigh usually has a roll pin that is pushed through the sides of the tail of the carrier through the weight, it is as simply as punching the pin out and removing the weight. This will need to be done to utilize the Law Tactical or Sylvan Arms folding adapters as they have an extension that fits into the tail of the carrier that gets utilized.​

Length of BCG and buffer matter

One needs to pay attention to the length of a 9mm BCG's in relation to standard 5.56 BCG's. There are 9mm BCG's that are shorter in length which can cause issues with breaking bolt catches on AR15's due to over-travel of BCG during cycling. This added over-traveled length provides enough additional momentum from the returning BCG towards battery that on empty magazine lock it can damage/break bolt catches. The fix is to take up the additional length through spacers (quarters fit nicely in the back of a buffer tube, stack enough to get the correct space taken up) or one can use a longer 9mm buffer that has been designed in it's length to accomodate for the shorter 9mm BCG's.

Why we need the correct reciprocating mass?

Too little reciprocating mass on blowback doesn’t allow adequate resistance (mass) to hold the detonating cartridge in battery so an out of battery (OOB) detonation can occur.

Too much reciprocating mass can cause “bolt bounce” where the momentum of the excess mass causes the bolt to chamber a round with enough residual force to bounce off the breech face and bounce. The disconnector in the trigger should prevent the sear to release but sometimes it is in just the right distance away an OOB detonation can occur.



This is what I've used and have provided to others I know that have assembled their own and has worked out great. If there is anything of the above that isn't clear or is in error to someone else's experience please respond. More than anything I want to get the correct information out there.

I didn't know where to put this whether in "Rifle" or "Pistol" sections but thought that "Gunsmithing and Repairs" fit; if the mods feel otherwise feel free to move it.

 

[JJFitch]

Good info! I'll be checking mine to see if I'm in spec here!

Smiles,

 

[Jim Watson]

A friend went 'round and 'round with one until he got a suitable spring for it, after which it shot just fine.

The situation is complicated by the existence of two subspecies of PCC shooters, maybe three.
The casual shooter uses standard bulk 9mm.
The hot competitor uses a lighter load in his PCC than his PCP for minimum recoil.
And I suppose there are some Tiered Operators who shoot +P.

A blowback needs to be "balanced" for the ammo.

 

[BreechFace]

A friend went 'round and 'round with one until he got a suitable spring for it, after which it shot just fine.

The situation is complicated by the existence of two subspecies of PCC shooters, maybe three.
The casual shooter uses standard bulk 9mm.
The hot competitor uses a lighter load in his PCC than his PCP for minimum recoil.
And I suppose there are some Tiered Operators who shoot +P.

A blowback needs to be "balanced" for the ammo.

Agreed, and a valid point that must be considered. Which is why I included the statement below.

Your comment on your friend getting a suitable spring for it is something that I do not have experience with, in my cases once I got the right amount of reciprocating mass, my carbine springs have always been fine. What was your friends determinations with changing his buffer spring, anything there that we can add to this to get better information?

I would welcome anyone that can add any information on buffer springs as it relates to blow back PCC's. I've always just tried to mess with one variable, buffer weight; but know that tuning can be accomplished with differing buffer springs as well.

Loads that one develops for the PCC should be considered (i.e. standard P, +P, +P+, etc) when determining which side of the scale of oz. above one is at.

 

[Jim Watson]

Beats me. All I know is that it is kind of an entry level gun of some off brand and he did not spend any great amount of money on it, none of the usual musical chairs bolt and Buffer stuff. I recall he said a Carbine spring was the fix.

 

[Rex in OTZ]

Sounds like much the same problems Ruger had with their .22 Magnum rimfire carbine.

 

[Kp321]

Many of the problems with PCC’s is mismatched parts. Carbines from reputable builders are usually balanced for factory ammo. Kits put together in the garage might not be. I shoot mine for fun and Steel Challenge so feed it ammo on the lighter end of the power scale, no problems of any sort.
One word of caution to PCC shooters. I swapped the trigger for a CMC unit which is my go-to AR trigger. The bolt in my PSA carbine would not reset the CMC hammer. A call to CMC revealed that they are aware of the problem and produce a special unit for PCC’s. Unfortunately, the carbine trigger is never on sale, bummer

 

[Archym]

I'm book-marking this forum for continued updates as I have just started stumbling down the 9mm AR PCC path myself. My main reason for doing this was all of the "Black (rifle) Friday sales" and the option to have what is basically two different instruments using the same lower receiver (kinda). My biggest condundrum is that I am using a rifle length stock and recoil spring. Most people seem to have the good sense to make a carbine using Carbine Size buffer tubes, springs, and buffers. However, with my ape-length arms, a short stock just is not a comfortable option.

I will be using an Anderson Arms UR; Mercury Precision Blowback 9mm bolt carrier Group; Ballistic Advantage 16" straight profile Barrel with just a basic knurled thread protector in the end. As stated above, I currently have the standard A2 rifle spring with a no name, standard rifle buffer weighing in at 5.2oz. I am almost sure that this buffer will not be heavy enough for the rounds I plan on using for this combination (5.5 - 5.7grains of Longshot, 115 grain Barry's Plated Round Nose bullet, Browning or PMC once fired brass, CCI 500 primer.) This instrument will be used 75% of the time at an indoor range with the longest distance shot being 25 yards. I don't like a lot of flash, bang, or recoil and I like to be able to get anything I shoot to make 10 holes touch each other (lovingly, of course) as close to the center of the target at my aging eyes will allow. Also, I don't shoot "double-tap"s, or even want to fire very fast. Ammo still costs too much and primers are still too rare/expensive to be blasting through my supplies too quickly.

The above load data is for a medium / somewhat light "Gallery" load that shoots quite accurately from a S&W 915 with a 4" barrel and even better out of a Springfield Armory Ronin with a 5" barrel. There are those on various forums that claim loads that shoot well in your pistol will be a good place to start from, maybe even a good load to stick with, in a 16" carbine.

I do have the option of both tailoring the loads I shoot, and I have a spare rifle length buffer (also weighing 5.2oz) that I can modify with a modest (older than dirt) Mill/Drill press setup. The BCG has not arrived in the mail so I don't have any dimensions on it's length or weight. Yet.

The next thing I would like to set up is a jig, press, SOMETHING that will allow me to accurately measure the pressure of the recoil spring at a given length of compression. It seems this measurement would be extremely valuable in ALL variants of the AR platform and I'm somewhat surprised that I haven't found any published data on that subject yet. Again why I'm book-marking this forum. Maybe one in our community does have access to that kind of data.

To those that contribute to this forum and all the others on this site, Thank You. This is a tremendous community and I'm thankful that I've both found it, and can contribute what little I can.

Be safe.

 

[nevadacal]

I'm book-marking this forum for continued updates as I have just started stumbling down the 9mm AR PCC path myself. My main reason for doing this was all of the "Black (rifle) Friday sales" and the option to have what is basically two different instruments using the same lower receiver (kinda). My biggest condundrum is that I am using a rifle length stock and recoil spring. Most people seem to have the good sense to make a carbine using Carbine Size buffer tubes, springs, and buffers. However, with my ape-length arms, a short stock just is not a comfortable option.

I will be using an Anderson Arms UR; Mercury Precision Blowback 9mm bolt carrier Group; Ballistic Advantage 16" straight profile Barrel with just a basic knurled thread protector in the end. As stated above, I currently have the standard A2 rifle spring with a no name, standard rifle buffer weighing in at 5.2oz. I am almost sure that this buffer will not be heavy enough for the rounds I plan on using for this combination (5.5 - 5.7grains of Longshot, 115 grain Barry's Plated Round Nose bullet, Browning or PMC once fired brass, CCI 500 primer.) This instrument will be used 75% of the time at an indoor range with the longest distance shot being 25 yards. I don't like a lot of flash, bang, or recoil and I like to be able to get anything I shoot to make 10 holes touch each other (lovingly, of course) as close to the center of the target at my aging eyes will allow. Also, I don't shoot "double-tap"s, or even want to fire very fast. Ammo still costs too much and primers are still too rare/expensive to be blasting through my supplies too quickly.

The above load data is for a medium / somewhat light "Gallery" load that shoots quite accurately from a S&W 915 with a 4" barrel and even better out of a Springfield Armory Ronin with a 5" barrel. There are those on various forums that claim loads that shoot well in your pistol will be a good place to start from, maybe even a good load to stick with, in a 16" carbine.

I do have the option of both tailoring the loads I shoot, and I have a spare rifle length buffer (also weighing 5.2oz) that I can modify with a modest (older than dirt) Mill/Drill press setup. The BCG has not arrived in the mail so I don't have any dimensions on it's length or weight. Yet.

The next thing I would like to set up is a jig, press, SOMETHING that will allow me to accurately measure the pressure of the recoil spring at a given length of compression. It seems this measurement would be extremely valuable in ALL variants of the AR platform and I'm somewhat surprised that I haven't found any published data on that subject yet. Again why I'm book-marking this forum. Maybe one in our community does have access to that kind of data.

To those that contribute to this forum and all the others on this site, Thank You. This is a tremendous community and I'm thankful that I've both found it, and can contribute what little I can.

Be safe.

 

[nevadacal]

To make a jig press you may be able to use a drill press & a pressure scale, bathroom scale may not be sensitive enough. Produce scale would be better. apply the same distance to the different springs. The weight reading being the gage. What is to light is just that, what's to heavy, ect. Trial & error seem to be necessary. I have the same concern for a 45 cal , ar build. Alot of no information. Different barrel length, bullet weight & powder charge or going to matter. Will be watching for your results, will post mine also. Merry Christmas to all.

 

[243winxb]

Old thread warning.

stumbling down the 9mm AR PCC path

The 9mm carbines, being a blow back design, can fire when the action is not fully closed. I suggest googling 9mm carbine kaboom.
https://www.thehighroad.org/index.php?threads/9mm-kaboom-yesterday.811544/
And https://www.thehighroad.org/index.php?threads/ar9-virgin-assembly-ka-boom.844227/

And https://www.thehighroad.org/index.php?threads/hipoint-40s-w-carbine-kaboom.773546/

 

[Lo-Profile]

I bought the ATI AR-9, very reliable and happy with it. Only thing I don't like is the lack of a last round hold open bolt.
Uses regular Glock magazines.

 

[243winxb]

Factory gun vs parts gun, 2 different animals.

 

[Atavar]

Good on you all. I see too many people building “kit” guns and ignoring the engineering that goes in to it. Designing and building a gun is no simple matter.
Many folks have gotten used to the nerf lined world we live in and expect any two parts that can bolt together to work perfectly out of the box.

 

[Blkhrt13]

I have been looking for parts to build my own carbine for a while thanks for this

 

[jmorris]

The next thing I would like to set up is a jig, press, SOMETHING that will allow me to accurately measure the pressure of the recoil spring at a given length of compression.

Something like this was pretty easy to make. The rod is threaded so the round eye can be removed and the spring removed/replaced with another. The Sharpie mark on the side is so they are all measured at the same compressed length. The 1x1 box tube has one side removed and the ends folded up with two holes drilled for the rod.

The fishing scale has a “memory” slide so you just pull to like the mark up with the bottom marker, release and read.



I have force gauges that are more accurate but cost $100’s more and the fish scale is all that’s really needed for this use.



For an AR buffer spring, maybe a sectioned piece of PVC pipe, longer steel rod and drill holes in two end caps.?

Not as quick to swap out springs for testing but if you just wanted to see what you have, how it holds up over time or have an AR and don’t want to build anything, just set the butt on a flat surface and pull with the scale until the locked back BCG just comes off the stop. Release pressure and read.



You can do the same thing with pistols.



Often, it is easier and cheaper to change the “weight” of the spring than the mass it is working in conjunction with.

Pistols like the one above also have other areas where function and feel can be changed without changing mass or the recoil spring. The main spring and even the profile on the firing pin stop will effect both more than many might imagine.

 

[BigBlue 94]

Been considering a 10mm AR for a while. Great info on reciprocating mass

 

[BreechFace]

Been considering a 10mm AR for a while. Great info on reciprocating mass

if you have a but more money than typical blowback, talk/check out Macon Armory (Rudy) out of Georgia for a direct impingement upper for 10mm or 45ACP. In my opinion on those two cartridges that would be the route I would go. Rudy makes a custom gas length tuned to those cartridges.

The 10mm utilizes a 6.8SPC bolt that he modified for the 10mm.

I was considering one but went with the Kriss Vector in 10mm for a pistol length PCC for something different than another AR.

 

[Archym]

Thank you again all who have contributed. Tuesday will be the first test of the now completed 9mm carbine upper. The Rifle Length buffer measures in at 5.3oz. The Bolt Carrier Group measures in at 14.6oz. This brings total reciprocating mass to (scratches in the dirt with a stick) 19.9oz. This is a little bit shy of the data that BreechFace listed in an above post. The spring in the lower is (obviously) a full length rifle buffer spring. Interweb Sources say that it should have 42 - 43 coils in it. Mine for some reason has about 44 coils. Taken out of the tube and measured it is 12.75 inches long. Again, I will be shooting "Lighter Than Factory" hand loaded ammunition in it for the first 2 rounds (I have 12 loaded). After the first two rounds, I'll take down the action, do an inspection, etc. My spotter will be checking empty shell ejection. This upper/lower configuration is basically just a safe queen and an indoor range instrument. Despite the truly silly looking quad-rail/tacticool hand guard, it's not meant to be some CQB/Survival firearm that will suffer through harsh conditions and extreme "Operations". I'll let you all know the results sometime later on in the week.

It may or may not be worthy to note that this particular lower, including buffer and buffer spring, only has about 100 rounds of factory Winchester and Remington .223 ammunition put through it and I was having trouble with ejection which some in this forum and others suggested might be caused be a buffer spring that is too stout. When the rifle was being built, I would not store the lower completely assembled. The spring was stored uncompressed. If this is the case, the 9mm carbine upper might be a good way to test out this theory and maybe even break in the spring a little bit.

I know I know..."Shoot more, theorize less." Good hunting to all. Stay safe.

 

[Armorer 101]

All of this is why, not needing one, and being an AR armorer for 40 years, I purchased a PSA Glock mag fed 9mm when they had them on sale for less than $500 I use the 124 Truncated Cone JHP with a real stout load in this gun, it feeds and functions. Put a red dot on it and gave it to my wife, she loves it….real light, no recoil, lots of boolets and she knows how to shoot. Being a PI from Memphis, she will pull the trigger….. Her old carry Chief Special is on the wall, got her an air weight Chief for her birthday to go with her CWP.

 

[Archym]

Not really sure how/why that last post fit into this thread. But it is always good to hear that a husband - wife team shoots well together. All the best to both of you, but maybe in a different forum.

 

[Archym]

I also have to post one more thing to help/warn/etc. When trying to go the "cheap route" and converting the AR lower using the MEAN magazine conversion implements, it really is crucial to use the PMAG magazines. I tried to use these inserts with a cheaper, non-brand 30 round magazine and after hours of creative dremel grinding, (and hammers) it still would not work. I wasted a lot of hours and a long drive to get an actual PMAG, 3rd gen, and everything slipped right in without a fuss. Dry fire testing (dummy boolits), and manually ejecting rounds seemed to work very well. If you're not going to spend $100+ for the glock or colt magazine insert and insist on using the "standard" AR type magazine, it really does have to be a Magpul PMAG. I don't like marketing or advertising for any specific company. I'm just trying to say what I've found to work. As always - Good hunting. Stay safe.

 

[Archym]

So last Tuesday's trip to the range was less than 100% successful. The MEAN magazine conversion did work flawlessly in feeding rounds. The main problem came with ejection. Shooting just 2 rounds at a time, the last bullet of the two shot groups was getting lodged into the fire control/trigger section of the inside of the lower receiver. After measuring the 5.56 bolt carrier group, and the 9mm bolt carrier group, the 3/4" difference in overall length became obvious. Many builders on various forums have identified this exact difference in length as the cause for empty shells to get lodged in the trigger area. It's possible that just making a 3/4" spacer and placing it in the back of the recoil spring could solve this. As this instrument has a rifle length stock, buffer, recoil spring, and buffer tube I have not been able to locate any manufacturer that makes an extended length A2/rifle buffer for 9mm conversions. The weight of the buffer did not seem to be a problem. Shooting lighter than factory loads resulted in very light recoil and 2 shot groups with holes very close to each other. I think I shot more accurately with the 6 malfunctioning shots out of the new carbine than I did with my .45 1911 that day. I probably will purchase another rifle length buffer and add some weight to it just for piece of mind. I'll post the results. Good hunting. Stay safe.

 

[Jim Watson]

I read of stacking quarters as spacers.

 

[Archym]

I got back from the range and the second outing with the 9mm PCC upper. The stack of quarters fix worked like a charm. I put about 20 rounds of lightly loaded 200gn copper plated round nosed bullets down the barrel and every one fed, banged, and ejected beautifully. No malfunctions at all. The bolt locked open after every last round from the MEAN-mag. I also put about 10 rounds of the same bullet with 5.1gns of CFE Pistol powder through it too. On the 6th shot of that group I heard a slight "Sproioioing" somewhere in the stock. Tilting the upper forward and pulling the buffer out showed that nothing was out of place. No scratches, no cracks, no scorch marks, no broken pieces. I only have a relatively inexpensive reflex sight on it for now. I was shooting three shot groups and after doing some adjustment, I was able to get the fourth three shot group to put three holes all lovingly touching each other in the X ring of a "trouble shooter" target at 20(ish) yards. I did notice that the ejected shells looked extremely black and sooty on one side. A quick scratch with a thumb nail would take most of it off, but I'm still kinda wondering. I've heard stories of how some SMG type firearms have such loose chambers that the brass is ruined after it's fired. I really hope that isn't the case with this carbine. I've got all my spent brass policed up into a separate bag and once it goes through the dish-washer and then the tumble polisher I'll mic it and see if there is any noticeable damage to it. Granted, this is brass that has already been reloaded 2 or three times.

I've got a second A2 rifle length buffer ordered. The plan is to drive the roll pin out of it, remove the rubber bumper and the stock weights, and cast some lead weights that will bring the weight of the buffer up to or maybe even over 6oz. I'm also going to try to cut some rubber spacers so that the weights don't "float" like they do in a mil-spec buffer. From what I've read, the floating weights were used to negate BCG "bounce" in full auto carbines. I'll never have to worry about anything full auto. And it seems like floating weights in a reciprocating mass would have a dead-blow hammer affect on the parts at the front and back stopping points. I'm hoping that keeping the weight in the buffer in a mostly firm position will negate that dead-blow hammer effect and seem more like a soft rubber mallet effect.

I've also ordered a spacer from New Frontier Armory that is supposed to be the properly engineered dimensions to eliminate the Bolt Carrier Over Travel Trouble. It looks like it's made out of plastic and it cost $6.95. Now don't get me wrong, I'm as cheap as a newborn chicken's first words, but I'm not holding out a lot of hope for the longevity of a seven dollar plastic piece at one end of the reciprocating mass of a blow-back 9mm bolt carrier group and porked up buffer. But if it does work well enough, it will give me the dimensions to fabricate a similar piece out of soft metal like brass or aluminum...provided my next door neighbor still has his mini-lathe. And he can get to it. And it still works.

All in all I will call today a win. Nothing blew up, caught on fire, spit shards of metal all over the range, or made me piddle myself. Once I get the new parts in place and do a little bit more testing with some other hand loads, I'll put up a little bit more well written post with pictures (of the parts), part numbers, measurements (again....of the gun parts), and maybe some load data of what has worked the best. I wish I would have taken some pictures of how the empty shells were caught in the trigger group down in the lower receiver before I put the quarters in the buffer tube. Maybe then others that do similar builds won't have that "heart in your throat" moment like I did. And yes...more piddling on myself. Maybe it's a good thing there were no pictures taken.

Thank you all who have contributed, chimed in, read along, or just put up with my long winded prattling as I've spelunked my way down the rabbit hole of this build. I would especially like to thank BreechFace for initially posting the valuable information in the Original Post. Hats off to you, Sir. I hope this thread stays alive so that others can share their experience and advice with similar PCC builds. I'll look forward to it.

As always - Good Hunting. Stay Safe.