Mine sheared off yesterday. New rifle (a build not factory). About 200 rounds through it.
I ordered a lost parts kit from Bushmaster that has 2 retaining pins and 2 retaining pin screws plus the take down pin and spring that the back plate keeps in place. Brownells had individual parts available also. Colt and DPMS.
Any idea what causes this? I have an H buffer in carbine stock. Cycled reliably up to now. Pin moved up and down normally to install/remove the buffer and buffer spring prior to breaking off.
Probably just a random occurance but thought I’d ask.
lahunter, yes there is a faitly deep indent on one of the two flat portions of the buffer face. It looks almost like it was peened. Definately a sharp strike with something to cause it. The other flat has a faint abrasion on one side but the edge is a bit thicker than normal, it’s flared out a bit. Glad you suggested I look closely. I was going to keep it but will replace it now. Th face of buffer looks like new.
masenomics, the bolt carrier slot at 6 o’clock and the back face of carrier look like new. No damage there.
When the bolt carrier returned to battery it did not quite lock up all the way. I reloaded and fired one more time before it happened again. So buffer was un-restrained for two shots. I finally opened rifle and realized what had happened.
PS I have a Syrac gen 2 adjustable gas block on this rifle and I usually shoot suppressed. I turn it down 2 clicks when shooting with the can. So I dont think I am over gassed, but wonder if I should try a slightly heavier buffer? this H buffer weighs 3.8 oz. I see that some are4.1, 4.6 and H2/3 over 5 oz. Any suggestions?
I think I may know what happened. I think your lower may be slightly out of spec. When you hinge the rifle closed, observe the interface between the tail of the bolt carrier and the buffer. The bolt carrier should make contact with the face of the buffer and push it back slightly, taking the pressure from the buffer off of the retainer. If your lower is out of spec then it could allow the buffer to impact the retainer and eventually break it. That’s why I asked what your buffer looked like.
With that being said, you don’t need a buffer retainer for the rifle to function and I suspect if you replaced it then it would only be a short amount of time before it broke again.
You’ll need to replace it to find out if that’s the issue though.
Great advice. You guys are better than a You Tube video!
I will need a new buffer retainer pin to trouble shoot.
This is a high quality billet lower by Bomberg but I think the stock extension tube may have been screwed too far forward. Whats left of the buffer pin still moved up and down but the edge of the tube looks like it is awfully close to the pin stud. I checked two other rifles and the rear of BCG does touch the buffer face when you swing it down to close. So I know what to look for now, and did not before.
Leaveammo to me. The rifle did not go into battery without the retainer pin holding the buffer face in position. thats why I knew something was wrong? Guess I can try it without the retainer pin once I back off the end plate and extension nut.
I am using a Colt upper, and there is some side to side play in the upper so I installed an accu wedge that tightened it up.
Get rid of the Accu-crap. The lower receiver extension should be screwed in until the lip of the extension contacts the lip of the retaining pin. It will ideally be about 1/3 of the way. The retainer should move up/down freely. You also need to ensure that the castle nut and endplate are torqued correctly and then staked.
I took my rifle to my NFA dealer and friend, tom Denall, TDF manufacturing. the tip of the retainer plunger was underneath the trigger assembly. Based on color (light gray) of the broken plunger he thought it had not been heat treated properly. He replaced it with a new one, spring was OK. When lowering the upper onto the lower, the rear of the bolt carrier did not contact the buffer until it was almost closed. compared to my other two rifles which contact 1/2 way down, it is not alot of pressure on the face of the buffer. It is a Colt upper. the castle nut was not torqued tight enough either. Back to the range. I will try without accuwedge but its a 3/4 - 1 inch ES rifle the way it is.
The buffer retaining pin hole is improperly located. There’s nothing that can be done to fix this. You’ll likely continue to damage your buffer face and beat up the retainers.
(no change of upper brand/make will make a difference on this.)
I had a retainer plunger sheer off, it was from a CMMG kit I had obtained during the panic figuring I would switch out later. The sheer area was suspect and I wondered if the part was even spec or treated properly also. I replaced with a part from a DD LPK and no issues since that was 3000 miles ago, I did not have any out of ordinary marks on my buffer or BCG, the lower is BCM and all parts torqued to spec, I triple checked that.
I either agree with Markm, or plunger failure, in addition IG advice to ensure those parts do not move and at that point working free and causing additional unwarranted stress on the retainer plunger.