Buffer and spring question

[Rittmeister]

Short version: what's the longest buffer spring recommended for a midlength-gas AR with a carbine-length buffer tube? And, should the buffer itself physically impact the inside back wall of the tube during normal function?

Longer version:

I have two parts-bin ARs. Both are midlength gas 16" barrel rifles. One, I put together as one of my first efforts; lower parts kit came from Palmetto and I followed the directions to assemble. Always worked great.

The second gun, I bought the lower used, already assembled, and put it together with another upper. It was a great deal with a Minimalist stock and Timney trigger already installed...

Both guns have a mil-spec, carbine-length buffer tube, as measured at 7.25" from the front of the threads to the back end, and no slanted back. Today for the first time I ran into a function issue with gun #2, where an unfired case got stuck in the chamber. I had to bring it home to clear it. At first I thought it must be an extractor problem but I took the bolt out and pulled the extractor and I can't find any wear/breakage on the claw, pin holes, or the spring. It also still looks just like the extractor on gun #1 (also took apart to check).

I got to thinking about how the two guns act differently when I retract the charging handle, and I pulled both buffers and springs to inspect those. Buffers are the same, both are carbine buffers (as googled) and weigh 3 ounces on my postal scale. However, the springs are way different.

Spring from gun #1 is 10.5 inches long. Gun #2's spring is 11.5 inches. When I retract the charging handle on #1, there's a definite "stop" at the back end, where I assume the buffer itself is hitting the back of the tube. #2 does not do this; it's much "softer" feeling, and it makes me think the coils of the spring are stacking and preventing the buffer from touching the back wall.

I googled a lot of this and got the standard "a carbine buffer spring should be at least 10" long" but I'm curious what length would be too long? I don't know if the longer spring is causing the feed/extraction issue but I tried hand cycling a mag of this ammo on the bench and had similar malfunction issues several times; then I did a mag in gun #1 with the shorter spring, no problem whatsoever, and when I put the shorter spring into gun #2 it similarly functioned perfectly. I have not fired either again though.

Sorry for the long-winded question. Any help appreciated.

 

[Varminterror]

Your spring length is dictated by your buffer tube length, not by your barrel gas length.

A carbine buffer tube uses a 38+/-1 coil buffer spring, 10.5” long when in good condition. A rifle buffer tube uses a 42+/-1 coil spring 12 3/4” Long in good condition. Under length springs with the right number of coils typically reveals the spring is over-worn. Under length springs with too few of coils are indications of hillbilly gunsmithing and should be replaced.

“Too long” for either spring really isn’t a thing. Too many coils is a thing - aka, using a rifle spring in a carbine tube. Stretched springs will typically “remember” their factory length after some time in the action. Some hillbilly gunsmiths think the springs can be stretched to gain strength - F=kx, increase x, increase F, but that would really require retempering of the spring. It works a while, then the spring comes back into spec.

The buffer should bounce off of the back of the buffer tube when fired. The bolt face should draw 1/4” past the bolt catch - if it doesn’t retract far enough, sand/cut the rubber bumper on the rear of the buffer to gain travel. If it travels too far, either replace the rubber bumper (if worn short) or stack washers in the rear of the buffer tube to limit the travel.

If the buffer doesn’t dead length against the back, it would leave the opportunity for the gas key to strike the lower extension bridge upon firing - aka, break your rifle.

 

[Rittmeister]

Interesting info. I've seen a number of spring length references but didn't count the coils at first. Both springs have 37 coils, indicating that they're both carbine springs. Perhaps the longer one was stretched. Clearly I need to order a new spring.

Thanks also for the info that the buffer should in fact impact the back of the tube - it is definitely not doing so with the longer spring.

If anyone has recommendations for a brand/model I'd be grateful. Also considering a heavier buffer to try.

Thanks again.

 

[MistWolf]

Order a Sprinco blue spring and an H2 buffer. Replace your extractor spring with either the Colt M4 extractor spring with insert, or the Sprinco M4 extractor spring with insert. Do not use an O ring.

 

[Varminterror]

I run H2 buffers in all of my personal AR’s, except my 3 Gun rifle (lightweight reciprocating system).

If your buffer is not making contact when the charging handle is pulled fully rearward, it MAY be caused by your spring coils stacking - hence the design for 38 springs, rather than 42. This isn’t great, as it will cause spring damage in time, but it’s not necessarily critically damaging to expensive action components. It may also be because your gas key is striking your lower extension bridge. This IS critically damaging to expensive action/receiver parts. Neither are good, but the former is certainly the lesser of two evils.