300 BLK Headspace Problem

Maybe try to sand/file the sharp edges and corners of the ramp, which the round may be getting hung up on. I had a similar problem but with factory and I honed the chamber and that seemed to work. Just a light honing so not to take off to much. I used these from brownell's https://www.brownells.com/gunsmith-...cessories/rifle-polishing-system-prod651.aspx. I bought 800 and 400 grit, the 800 worked for me.

Shouldn't that sizing die be screwed down to touch the shell plate?

Max SAAMI COAL is irrelevant for most bullets. What OAL are you using? Does your sized brass sit in the chamber OK? I’d guess the bullets need to be seated deeper.

Test fit in your chamber not the guage. If a sized piece fits then move forward in your process until it doesn't and you will find where the problem is. Hand loading does require basic troubleshooting at some point so become familiar.

Did you compare the measurements of your cartridge with a factory one to see what’s different?

AR-15 SBR fires factory ammo fine, but half my reloads, all from the same batch, jam and won't chamber.

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When you say "jam and won't chamber," where does the jam occur ?
- Right as the shell enters the chamber ? (early jam)
- or in the last 1/8" or so ? (seating jam)

- Does a resized (but empty) case fail to lock up ? (sizing problem)
- Or does it freely go into full battery when the bolt closes? (sizing's fine, something happening upon bullet seating)

- Does a loaded -- but uncrimped -- round lock up OK ? (you have a crimp-while-seating bulge problem)


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Start with just the brass, size and see if it fits. If not keep lowering the sizing die till it does. Once you have that set, lock it in. Now try a couple more to make sure. Next step is to seat a bullet, NO CRIMP and see if it chambers. If not determine where it's hitting and keep shortening till it chambers. This will be your MAX OAL with this bullet. I prefer to have a min of 0.020" jump to lands. So I lower the seating stem till I get what I want. If your bullet has a cannalure and it's in the right place to crimp, you can add a crimp. Next if you crimp with the seating die, back off the seating stem, run the ram up and die down till it contacts. Back the ram down, turn die body down 1/4-1/2 turn. Lock then run the bullet up into the crimp die to crimp. If not enough., add another 1/8-1/4 turn. Ckeck again. If good now run the round back up into the seating die then run the seating stem down till it contacts the bullet and lock in place. You now should be set.

If you over crimp you can buckle the brass just below the shoulder. You can paint the brass with a marker to check for contact if it fails to chamber.

Please report back with your findings!!


Thanks for taking the time.

Stupid question but did you gauge the brass AFTER seating bullet? You can very slightly squish the shoulder and cause condition you're describing - especially if it's intermittent. No inside mouth chamfer or rough trim operation + flat base bullet, can put a tiny bulge where the case starts to taper into the neck and it's very easy to not see it. This is an exaggerated example (middle round), but you can do that very slightly & get random rounds that won't chamber & easy to not notice when handling the finished rounds.

Edit to add: what specifically does JAM mean in your case? What I pictured will feed just fine but bolt won't close and if you have a strong enough buffer spring or try forcing into battery with forward assist, it will stick in chamber & need rodded out.

If JAM means they fail to feed, a 110 VMax under mag length should get gobbled up. So if that's the case - depending on what generation PMag you're using and if you shaved the rib a little if an early generation - try a Lancer L5 AWM. Those are excellent feeding 300BLK. From there, check barrel extension clocking, feed ramp burrs, etc

Recommended oal's with 110 V-Max are 2.00" to 2.040". I've seen some data sources list much longer oal's. The neck diameter measured after bullet seating should be .334 or less. Some converted brass have too thick case walls and necks will be too large. My P- mags, don't know what version, leaves rounds in the mag loose where they can slide forward and back? I just don't use P-mags and prefer the military aluminum mags.

Some good info here from above posters, but I have a different way than some to find the cause of these errors

First thing I would do is measure a piece of factory ammo that does chamber with a comparator. This will tell you if your shoulder is bumped back far enough.
Next I would check to see if a case chambers in your gun. Clean the chamber thoroughly first. Take a Sharpie and color the case around the neck and shoulder on a sized piece of brass. Drop in the case and slingshot the bolt home. Extract the case. If you see a ring around the shoulder, then you know your shoulder needs more bump.

Now that you have figured out your shoulder bump, load a dummy around. Load a bullet in an empty case that will chamber to the desired COAL. Color the bullet with a Sharpie. Drop it in the chamber and slingshot the bolt home. Extract the case and take a look at the bullet. You will see marks on the bullet if you are touching the rifling. Decrease OAL if you see this. Also, if your brass neck is too thick, you will see the marker worn off the neck. This is actually a somewhat rare situation, but can happen.

Lastly, I see you are using RCBS small base dies. They will form brass correctly. Lee dies are known to not form 300 BO correctly. The small base may not be necessary, as they do work the brass more. Your mileage may vary on this one!

Ahhhh, the beloved 300BLK, probably my personal favorite round. Out of every cartridge I load for, this is simultaneously the simplest, and yet trickiest to make it work in multiple guns. I currently have 4 guns in the caliber, and I load and shoot about 10 to 20K rounds a year in this. After having messed with this caliber for several years now, here's where I've settled:

1. Small, light bullets (~110 grain, etc)....generally should be loaded short, like around 2.05 and sometimes even a little less. These generally run fine in standard GI mags as well, as there is no conflict with the ridges in standard 5.56 mags.

2. Small Base Die Set can really help a guy out when loading primarily for the AR platform, but isn't necessary. But you REALLY have to be johnny on the spot here........unlike what most reloaders are used to, YOU CAN'T JUST CRANK YOUR SIZING DIE DOWN UNTIL IT HITS THE SHELL PLATE. I should maybe say that again? You need to set the die up with a case guage. Go down until the shoulder bump is correct, and your brass fits in the guage flush or slightly above the LOWER level, and below the UPPER level. You can use the chamber of your gun, if that's the only AR you're ever going to shoot it out of, but that makes it hard to tell if your shoulder is bumped to far.

3. A common issue I've seen a few people have with 5.56 and 300 BLK, especially when using cut 5.56 brass they formed into 300 BLK themselves is to crank the die down to achieve plunk, but end up with too much shoulder bump.

4. If you are plunking correctly, between the levels on the gauge, Then to me.....my next step is to look at mags, crimp, and then finally, the gun. Does your gun work with the first few rounds, but only has problems when you get down the mag? Or is it consistent? You said factory ammo ran fine? Was it the SAME bullet? Or another bullet with a different profile and weight? Did you try multiple magazines? Interestingly, I personally have better luck with old school GI mags with the small bullets. I have one AR that will not run 110s in a 300 BLK Pmag, only a gi mag, and that same gun won't run 220 subs out of a gi mag, only a 300BLK Pmag. My other AR will eat anything I throw at it, with any mag, and never miss a beat.

KentM said:

Rounds fit the RCBS chamber gauge fine, which. . .

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would be relevant if you were shooting the chamber gauge. You aren't. Loose the chamber gauge, and use the one built into you're rifle.

As a starting point, set the RCBS SB FL die to a light cam-over, size a few cases, and see if the cases chamber. Load them, and check again. You'll be able to separate sizing problems from seating problems, and we'll be closer.

In a bolt action I would suggest partial FL sizing to fit your chamber, but in an 300BO AR, I think it's unlikely to be worthwhile.

KentM said:

I guess I could just keep cranking down the RCBS Small Base resizing die until the shoulder bends. . .

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You are confused about the meaning of at least one of the words you've used in this sentence. It's not possible to "bend" a "shoulder" with a "resizing die".

"...the trigger would go "click" and when I tried to clear the weapon the charging handle
was jammed and had to be mortared to get the shell out. Therefore I have no way of
knowing exactly where the shell was when the jam occurred."

Click to expand...

That is classic shoulder buckle.

I am not crimping - merely tapering with the RCBS bullet seating die.

Click to expand...

Are you seating/"taper" crimping in 1-step ?
If so, you are still buckling the shoulder.

Loosen the seating die (body) to the point their is no contact with the case mouth,
then adjust the seating stem to correct OAL. (You do not require crimp of any kind
with anything approaching normal/standard neck tension)

Try that.





postscript: did you yet test:
- Does a resized (but empty) case fail to lock up ? (just hand feed the empty case/let the bolt slam home)
- Or does it freely go into full battery when the bolt closes? Can you easily eject the case manually?
(sizing's fine, something happening upon bullet seating)

Just as an aside - I've had issues with 300AAC when using reformed / 'processed' 5.56 NATO brass. The 'reprocessing' doesn't seem to reliably correct for fact that the parent 5.56 NATO / 223 case has thicker brass walls down-body, and as a result it's pretty common to see neck walls on reprocessed brass that are way thicker than spec. In these instances, the sized brass will fit a gauge but once a bullet is seated - the neck's OD is too large to fit in a SAAMI chamber.

KentM said:

(From the OP)
OK, so many responses to reply to. Thank you all.
The RCBS small base die was installed per instructions. The loaded cartridges fit fine into the RCBS case gauge and when mic'ed they are well within SAAMI specs for length and diameter.

I am not crimping - merely tapering with the RCBS bullet seating die.ight

When firing the ammo, some would fire properly, then one would jam. By that I mean the trigger would go "click" and when I tried to clear the weapon the charging handle was jammed and had to be mortared to get the shell out. Therefore I have no way of knowing exactly where the shell was when the jam occurred. Looking through the ejection port just shows a cartridge in the chamber (fully chambered or not?), and another from the magazine backed up against it. COALs are 2.050 to 2.060 with a SAAMI max COAL of 2.260.

Dummy rounds (just the projectile no primer or powder) jam similarly when I try to cycle the charging handle manually. Some work, some don't,, but no difference in mic'd measurements.

I have tried the "plunk" test dropping a round into the chamber, but with the limited access in an AR-15 it isn't easy to see how it seats. Unlike a pistol, you can't feel for space or rotate the case with your finger. ** Is there some technique here that I've missed? **

Click to expand...

I would say you could have one of two things going on. One, if you are expanding the brass (you don't need to shooting jacketed BTW, only if your running cast or plated), and you are not crimping it enough.......that could cause the round to not fully go into battery....and you get the click. Second possibility.....you're bumping the shoulder way to much........the round is seating to deep, that could cause the "click" and having to bang it out. You definitely need to take a very close look at your case guage. You should have two distinct levels on the end you put the round in...a groove cut across it. The very top gauges your max OAL. The rim should be flush or ***slightly*** below that. The lower part is the minimum.....your rim should be flush or ***slightly*** above that. In all cases it must be between those two levels. I use a small straigh edge....put the case in........run the straight edge over the whole case guage, you should NOT feel the rim. Next, line the straight edge up with the groove and slide it back and forth......you should be able to slightly feel the rim. If you see ANY light, or you can get a feeler guage between the straight edge and the rim of the case........you've set it back way to far and need to back your die up.

KentM said:

(From the OP)
OK, so many responses to reply to. Thank you all.
The RCBS small base die was installed per instructions. The loaded cartridges fit fine into the RCBS case gauge and when mic'ed they are well within SAAMI specs for length and diameter.

I am not crimping - merely tapering with the RCBS bullet seating die.

When firing the ammo, some would fire properly, then one would jam. By that I mean the trigger would go "click" and when I tried to clear the weapon the charging handle was jammed and had to be mortared to get the shell out. Therefore I have no way of knowing exactly where the shell was when the jam occurred. Looking through the ejection port just shows a cartridge in the chamber (fully chambered or not?), and another from the magazine backed up against it. COALs are 2.050 to 2.060 with a SAAMI max COAL of 2.260.

Dummy rounds (just the projectile no primer or powder) jam similarly when I try to cycle the charging handle manually. Some work, some don't,, but no difference in mic'd measurements.

I have tried the "plunk" test dropping a round into the chamber, but with the limited access in an AR-15 it isn't easy to see how it seats. Unlike a pistol, you can't feel for space or rotate the case with your finger. ** Is there some technique here that I've missed? **

Click to expand...

Like I said in my first post, I had a similar problem with factory and I honed the chamber with 800 grit and it fix the issue. Chambers can have high spots, are not as smooth or slightly undersized. Sounds like you are doing everything correct on the reloading side. IMHO

rbernie said:

Just as an aside - I've had issues with 300AAC when using reformed / 'processed' 5.56 NATO brass. The 'reprocessing' doesn't seem to reliably correct for fact that the parent 5.56 NATO / 223 case has thicker brass walls down-body, and as a result it's pretty common to see neck walls on reprocessed brass that are way thicker than spec. In these instances, the sized brass will fit a gauge but once a bullet is seated - the neck's OD is too large to fit in a SAAMI chamber.

Click to expand...

This is very true.......there's a 300 BLK page with listed brass and their specs to check go/no go for converting to 300 blackout. I generally prefer to convert Lake City, and most of my converted brass is LC head stamped. The PMC brass I've converted has exactly the problem you reference, and how I learned about this little piece of trickery.

Isolate the problem:

Did you yet test:
- Does a resized (but empty) case fail to lock up ? (just hand feed the empty case/let the bolt slam home)
- Or does it freely go into full battery when the bolt closes? Can you easily eject the case manually?
(sizing's fine, something happening upon bullet seating)