308 BCG hitting lower

LARUEminati

LARUEminati

Active member


As you can see from the pic my BCG is coming back too far in the buffer tube and hitting the lower, and from what I’ve read this seems to be somewhat of a common issue with 308’s in regards to the buffer itself being slightly too short. Has anyone here had this or seen this issue that can chime in?

Apparently since there are no set-tolerances for AR-10’s there can be a variance in the buffer length/tube length/bcg overtravel. I saw some pics of various brands of lowers showing this marring. I’ve read the quick fix is to either 1. Add a quarter or two inside the buffer tube to shim the spring forward slightly and reduce the inside length of the buffer tube, so when the buffer comes back it can’t go far back enough for the bolt to hit the lower. 2. Trim the lip off of the buffer tube so you can get one more full turn in, thus shortening the buffer tube and essentially receiving the same result as adding the spacer inside the buffer tube would. 3. Buy this fancy $100 slash buffer which is apparently slightly longer than the DMPS one and is made to solve this issue.

I was able to come across some measurements on how to check this. With the buffer installed, no spring, and the BCG sitting ontop of the buffer with the bolt fully extended their should be anywhere from .120” - .187” between the face of the bolt and the bolt catch. Mine with the standard DPMS buffer installed and the carbine length buffer tube was at .220”. I added one quarter as a shim and it brought me to .136”, which is what I’m going to try out first.

All parts are Aero Preicison, upper/lower/nickel boron bcg/buffer kit/lpk. I built it as a cheap lightweight deer rifle.

Man, what a wacky problem. I guess that’s why you buy complete’s and not build budget guns.
 
Why don't you try a Tubbs flatwire spring. I don't have an AR10 but run them in my AR15 and really like them gives you more dwell time and buffer doesn't travel as far.
 
Originally Posted By: bryan hWhy don't you try a Tubbs flatwire spring. I don't have an AR10 but run them in my AR15 and really like them gives you more dwell time and buffer doesn't travel as far.

Well that was my initial thought, but from my understanding regardless of the spring the buffer is supposed to contact the back of the buffer tube. I get the fact it will soften the blow to the plastic tip end of the buffer, but i'm not so sure the spring will correct this and not cause harm to my cyclic rate which is currently pretty good.

I'm no gunsmith so in reality I have no clue, figured someone else might have seen this before. I do have access to a heavier spring which I will definitely try if the shim doesn't fix it. (the spring is not local, I'd have to drive a ways to go pick it up.)

Thanks for the input.
 
What part of the carrier is hitting? The bottom bearing surface? Sounds like the shim you added should do it, crappy production tolerances look to be the blame.

How is the gassing on this gun?
 
What part of the carrier is hitting? The bottom bearing surface? Sounds like the shim you added should do it, crappy production tolerances look to be the blame.
Where the BCG flares out at the "guide rails", above the circular portion that first enters the buffer tube. It's impacting that area of the lower about .060".

How is the gassing on this gun?
It may be overgassed but that's TBD on my next outing. I'd like to think I have a good feel for AR's and it felt like it was running good. No adjustable gas block i'm just running a standard lo-pro gas block. ran 100% with aguila 150's, had one double feed with PMC's so I think it's running decent. Ejection is at 1:00 which could be a sign of overgassed but the upper also has a huge brass deflector so i'm not sure if thats contributing to the ejection angle. Once I start reloading for 308 i'll be able to tailor the load, i'm hoping.
 
I think it's over gassed and slamming the back of the buffer tube and a flat wire spring would help it should not be ejecting at 1 maybe even a adjustable gas block would be good you can order the flat wire from alot of places.
 
1 O' clock is seriously over gassed. Drill and tap your gas block if you are capable. I would start there. With a extreme ampunt of gas yes its most likely compressing the buffer pad enough to kiss the tube.

What length of gas system and how big is the gas port?
 
Originally Posted By: Widow maker 2231 O' clock is seriously over gassed. Drill and tap your gas block if you are capable. I would start there. With a extreme ampunt of gas yes its most likely compressing the buffer pad enough to kiss the tube.

What length of gas system and how big is the gas port?

Ok so heres the update, I tried the quaters for shims and two of them got the gun running good and the bolt wasnt hitting the lower. I was getting double feeds before with 308 pmc's and 308 federals and I think those were the two that were sending the bolt too far back. With the two quarters both rounds ran without a hicup and the bolt wasn't hitting (i put marker over the shiney metal to see if it was hitting), with one quarter it was still hitting causing the double feed. The aguilas that initially worked 100% must have been light loads not pushing it back far enough to hit.

Also, with the two quarters the ejection corrected to 3:00-4:00. It did feel a little rough. When I first had the double feeds before the quarters I thought that was from overgassing, but it must have been from the bolt hitting because once that was corrected no more double feeds.

The gas system is mid length, i'm not sure what the gas port is. It's a rainier select polygonal which I believe is made by BHW. I just got a plain jane mid length gas tube and block from rainier. Factory dpms spring and buffer (supplied by aero precision). The muzzle break is pinned so I dont really have the luxary of swapping the gas block.(commie state laws).

I've read of the quarters thing working and it apperas to have worked in mine for now, but I also read somehow after people did this they started breaking bolt catches. Well guess what, when I got home after I thought the gun was 100% I see a small chip in my bolt catch. Might have to start messin with the spring & buffer. What a mess
 
Since your gas block will be a PIA to get off try a flatwire spring like Bryan suggested. Commy laws are ridiculous.

The quarters must be stiffening the spring enough to make the bolt return to fast just grazing the bolt catch upon feeding. Without them the bolt was flying back too hard. You went from one extreme to another all with $ .50.
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Feeding and extraction is a delicate dance. One thing greatly effects the other. Iv really learned that building blowback guns. You could try making a carrier weight. Iv made a few for AR-15's out of grade 8 bolts, like the Tubb carrier weights. 1.3oz makes a huge difference to reciprocating mass. Slows the bolt down when you need to tune things and dont want to mess with the gas block.
 
I did contact Aero Precision, they said they would take a look at the gun if I was willing to send it back to them. However given whats going on in MA right now i've got advice from a few people including my FFL not to let the gun leave the state.

Aero also offered to replace the lower, but the new lower would have a different serial number and I wouldn't be able to get that transferred in. They did not offer any remedies as if they have fixed this issue before, even though I sent them links to other threads with the same thing happening. Pretty much a perfect storm i'm experiencing.

Does anyone think if I were to switch to an adjustable gas block, that I could just fiddle with that until I got the gas right without screwing with the buffer and spring? Or am I better off compensating with a new buffer and spring? I spoke with the guy that pinned my break and he said he could get it off if need be. I have an orange spring from springco on order, it says it's made for 308 gas guns so that may help.

Like I mentioned above the gun cycled perfect with the $.50 but it did still feel overgassed, it is a 308 though and i'm used to rockin .223's.
 
Originally Posted By: Widow maker 223An adjustable gas block gets to the root of the problem, the spring will "help".

Just ordered an SLR7, mine as well just do it and be done with it. In hindsight I should have gone adjustable right off the bat.
 
The bumper on the buffer is supposed to act as the dead stop for the BCG travel. If it's too short or too soft, OR the action over gassed or under sprung, then that contact happens.

Shims in the receiver extension work just fine.

Fixing the gas flow is the proper fix. 100%.
 
Well it appears the adjustable gas block and new spring worked.

I've got the SLR gas block on setting 4 which really showed me how overgassed it initially was. On setting 4 with the stock spring it was functioning fine but still ejecting to 1:00. I put the new sprinco orange spring in and it functioned perfect and ejected to 3:00-4:00 with ALOT softer recoil.

Hopefully all is good going forward. Appreciate the input.
 


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