Carbine spring/buffer in a rifle tube?

[Elkins45]

I have a 9mm AR that has what I'm told is a spring and buffer dedicated to the 9mm blowback bolt operation. The gun has a collapsable stock but I would like to replace it with an A2 fixed stock. Is there any reason I can't just move the carbine spring and buffer into the rifle tube?

 

[dogmush]

The buffer will be too short, and will allow the bolt carrier to go too far into the receiver extension before bottoming out.

Since this is a 9mm buffer, and they are longer than a standard carbine buffer, it might bottom out before the carrier key hits the charging handle (It might not, you'd have to check) but it will definitely allow the carrier to pick up a little speed before it hits the bolt catch on the last round, and could cause bolt catch breakage. (That's what the longer 9mm buffer is used to prevent).

Folks (not me) have reportedly had success with using a pile of quarters in the bottom of the receiver extension to "tune" where the buffer bottoms out, and there are aftermarket spacers that do the same thing.

Basically, if you want to use the same buffer and spring, You'll need some kind of spacer to make the internal depth of the rifle extension the same as the internal length of the carbine extension. There's about 2 7/8" difference in the external length of those tubes. (rough measurements)

 

[badkarmamib]

I don't know if you would be interested, but Magpul makes a fixed stock that fits over a mil-spec carbine tube.
http://www.academy.com/shop/pdp/magpul-moe-fixed-carbine-stock-201818232--1
Then you wouldn't have to change the tube, buffer, or spring. Obviously, this is worthless info if you are trying to achieve the classic look, but I figured I would mention it.

 

[Elkins45]

The buffer will be too short, and will allow the bolt carrier to go too far into the receiver extension before bottoming out.

Since this is a 9mm buffer, and they are longer than a standard carbine buffer, it might bottom out before the carrier key hits the charging handle (It might not, you'd have to check) but it will definitely allow the carrier to pick up a little speed before it hits the bolt catch on the last round, and could cause bolt catch breakage. (That's what the longer 9mm buffer is used to prevent).

Folks (not me) have reportedly had success with using a pile of quarters in the bottom of the receiver extension to "tune" where the buffer bottoms out, and there are aftermarket spacers that do the same thing.

Basically, if you want to use the same buffer and spring, You'll need some kind of spacer to make the internal depth of the rifle extension the same as the internal length of the carbine extension. There's about 2 7/8" difference in the external length of those tubes. (rough measurements)

I wasn't aware that was why the 9mm buffer was different.

Could I just use the standard rifle buffer?

 

[dogmush]

I wouldn't were it my gun.

A 9mm buffer does two things differently then a 5.56 buffer. One, as I mentioned, is it limits the bolt cattier's travel rearward so that the carrier isn't moving as fast when it hits the bolt catch on after the last round. Second, they tend to be heavier. 9mm AR's are straight blowback, so the total weight of the bolt/buffer assembly (mass really, but you follow) controls how quickly the bolt opens after firing. Too light a buffer and the bolt opens early, while the chamber pressure is too high. In extreme cases case failure results. (There are other variables; barrel length, spring rates and ammo loading being the main ones)

I run a long, 8 oz buffer in my 9mm AR, and it works great.

A rifle buffer is both too short (in a rifle extension, for a 9mm bolt) and only 5.2 oz. The very lightest commercial 9mm buffer is 5.5 oz, and most are 6.5 to 9 (ish) oz.

I built my 9mm from parts, and dislike brass fragments to the face, so I went heavy, and figured I could back off the buffer if the carbine didn't function. As it turned out I didn't need to.

In your situation, assuming your gun functions well now, I wouldn't mess with the bolt/buffer/spring. Buy an A2 stock and receiver extension. Measure your curent tube and the new one, on the inside, from the back wall to the edge. Find the difference. Get a piece of solid aluminium round stock that just fits inside the tube and cut it to the length needed to make up the difference in the extensions. Then mount the new stock and put spacer->old spring->9mm buffer, in that order, in the receiver extension and rock on. That way the bolt and buffer will be reciprocating in the same space, at the same speed they are now. You'll add a couple ozs to the back of the rifle, but I'd bet you'll be hard pressed to notice them.

 

[Elkins45]

This is very good information and I thank you for it.

Since the 9mm setup doesn't need the reciprocating mass to prevent bolt bounce, why not just open up the back end of a rifle buffer and pour it full of lead? I think my rifle could benefit from some added bolt mass even though I'm shooting it with the buffer and spring it shipped with as a dedicated 9mm unit. I say that because of the snappiness of the recoil, plus a very swollen case I picked up after the last firing.

 

[Ibmikey]

As suggested earlier if you want a fixed stocck Magpul makes a fixed carbine stock that works just fine.

 

[dogmush]

This is very good information and I thank you for it.

Since the 9mm setup doesn't need the reciprocating mass to prevent bolt bounce, why not just open up the back end of a rifle buffer and pour it full of lead? I think my rifle could benefit from some added bolt mass even though I'm shooting it with the buffer and spring it shipped with as a dedicated 9mm unit. I say that because of the snappiness of the recoil, plus a very swollen case I picked up after the last firing.

You still don't want bolt bounce on a 9mm, but it's secondary to the other things the mass of the bolt and buffer are controlling. Remember: It's blowback. Too heavy and it won't cycle at all. Too light and it'll open early. The inertia of the bolt/buffer combo is what controls the timing on a blowback AR.

I haven't done the math on this particular combo, because commercial 9mm AR's exist, and the engineers did it for me. If you're getting bulged cases then you might very well be running it a little light. Whether a rifle buffer full of lead would be the right weight would require a scale and a calculator. (I guess you could do it by volume and density of lead, but that would be more math)

My advice:
Weigh and measure the buffer you have. figure out which variant was shipped to you as "9mm". Not all manufacturers are the same. Closely observe several pieces of brass for signs of early bolt opening. If it was just one bulged case, probably something else. If it's a bunch of them you might have to tune your rifle. Armed with that information you (or we) could estimate what you need to do to get the performance you are looking for. Without it, you are just throwing random parts at the rifle and hoping for the best.

On the stock, most of the 9mm AR recoil systems are set up for a carbine length tube. So it will be an order of magnitude simpler to put a spacer in the rifle tube to shrink it down to the carbine length, rather than try and re-engineer something for the rifle length tube. I'm ignoring the "fixed stock on carbine length extension" options because you specified you wanted an A2 stock.

 

[Elkins45]

The gun is a PSA with a Glock magazine dedicated lower. The buffer is the one PSA sold with the lower. It's 3-3/16" and weighs 5.5 ounces according to my kitchen scale. It doesn't have any reciprocating mass.

I already have an A2 stock and tube, so turning a spacer from aluminum stock would be a simple way to use the existing spring and buffer. That doesn't do anything about the recoil/early unlocking however. I could very easily add mass to a rifle buffer a couple of ounces at a time and then melt out a bit if it stops coming back far enough to pick up the next round.

I just don't want to have any more of these:



I enjoy having fingers and a face too much to risk a case letting go while the bolt is still open.

As suggested earlier if you want a fixed stocck Magpul makes a fixed carbine stock that works just fine.

Thanks for the suggestion, but I already have an A2 stock and tube that came off of my Colt SBR.

 

[dogmush]

OK. Now we're getting somewhere.

That is either an H3 Carbine buffer, or something very similar to an early Colt commercial 9mm buffer. I'm not Colt, but I would run something heavier.

So a good plan, if you're up for it, would be to install the A2 setup on the rife, then open the rifle buffer up and add weight until you get it to ~8.5-9 oz. (including the polymer bumper) For comparison, a Spikes 9mm buffer is just over 8.5oz and Slash's heavy 9mm buffer's are either 8.5 or 12 oz. That's step one.

Step 2 is assemble the whole thing, and retract the bolt all the way till the buffer bottoms out in the receiver extension. Observe the distance between the bolt face and the bolt catch. Compare that distance with the same measurement on a 5.56 gun and the difference will be the amount you need to limit the rearward travel of the bolt. then turn up something like this or this that is the right length. Assemble the whole thing with a wolf extra power buffer spring and you should be GTG.

 

[Walkalong]

so turning a spacer from aluminum stock would be a simple way to use the existing spring and buffer.

I did it in some sort of plastic/whatever. I forget what weight buffer is in there, but it came stock in the 9MM RRA AR. (I don't remember changing it.) No bulged cases, but it slings brass a long way compared to pistols. You can get longer ones now that don't need a spacer, in several weights.

 

[Elkins45]

I added a rifle buffer to yesterday's Brownell's order (on sale for $7.99) so I can pop out the steel disks and pour in some lead. I think I will start by taking it up to at least eight ounces.

I have a fairly thick Teflon rod in my scrap pile that might make a good travel limiter. I will revisit this thread with results once I get everything done and tested.

 

[Quentin]

The carbine 9mm buffer and the rifle buffer both weigh 5.5 oz but as said earlier you must use the rifle buffer in a rifle tube and the shorter carbine buffer in the shorter tube. That makes me wonder if you could use the rifle tube and rifle buffer on your 9mm, depending on the strength of the rifle recoil spring.

 

[Onmilo]

The carbine 9mm buffer and the rifle buffer both weigh 5.5 oz but as said earlier you must use the rifle buffer in a rifle tube and the shorter carbine buffer in the shorter tube. That makes me wonder if you could use the rifle tube and rifle buffer on your 9mm, depending on the strength of the rifle recoil spring.

Actually, you CAN use a carbine buffer in a rifle length extension tube but you MUST use a rifle length extension spring along with it but with the weight of a rifle length buffer so close to the heavier carbine buffers the rifle length buffer will work just fine,,, with a RIFLE length spring,,, in a RIFLE length extension