A buddy of mine just finished building a 7.5" SBR. We took it to the range to test fire it. I fired 1 round and the bolt locked to the rear and the case was lying on the top of the magazine. I switched the H buffer out to a standard CAR buffer and it did the same but had a different “feel” so i loaded 2 rounds in the Pmag and it cycled the first one and the second one was lying on top of the magazine like the first time. I then loaded 5 rounds and fired one round and the second one up entered the feed ramp and got jammed in with the spent case half way out of the chamber. I then took my bolt and carrier ( Colt semi auto with a cut away rear) out of my 11.5" and put it in his gun. The gun ran flawlessly. I put the H buffer back in and we continued to shoot it over 200 rounds without ANY problems(mostly rapid fire). We used several types of ammunition including 55 gr Hornady TAP, 55 gr Federal XM 193 Tactical, 55 gr Remington green box, 62 Gr Federal American Eagle, 68 gr Hornady BTHP and some reloads. No malfunctions what so ever.
Here’s the issue… We lubricated the original bolt and carrier and put it back in the 7.5" gun. It failed to function the same way again. I then placed this brand new bolt and carrier in my 11.5" gun and it ran a 30 round Pmag without any malfunctions. The new bolt is a Bushmaster carrier (that needs to be staked) and a bolt from MA Parts. It had an upgraded extractor (black) but it did not have MP stamped on it. It is obvious that my buddy needs to purchase a new bolt but why did the one work in one gun and not at all in the other? My bolt carrier has less mass at the rear because it is cut. I recommended a BCM (not in stock) bolt or a DD (in stock) but if it is bolt weight, is this the right choice?
Grant, GotM4, RobS,Gunz??? Can you give some imput??? PLEEEEZZZZZEE??? 
It was kinda hard for me to grasp totally what you were trying to explain. But your BCG worked in his rifle correct? And his BCG worked in yours but you only fired 30 rounds correct? Did you try to just swap carriers around while using the original bolts in each rifle?
so his carrier worked in your gun, and yours worked in his? and you swapped the complete BCG between guns, not just the carrier?
7.5" guns are very unforgiving.
It may be that the extractor on his bolt does not have a sharp edge to the hook and is slipping. It may also be that the spring and insert (although black) are not the correct ones and aren’t providing enough tension.
I would purchase a BCM extractor spring upgrade kit. Swap the spring and insert and try again. If still no go, replace the extractor with a new one. If still no go add the o-ring.
Your buddy would still be well werved by picking up a new bolt or complete bcg. A BCM, DD, or LMT would be my choices.
Good luck!
-RD62
Yes, I only swapped the entire BCG not just the carrier. I was weary of the whole BCG set up he had so I just swapped the whole unit. I only fired 30 rounds of his bolt in my gun because that was all the ammo we had left. HE has not purchased a dedicated BCG for his gun yet. We used his carrier that was brad new and a bolt that we use to do head space checks. We keep it in the kit with the extractor removed. I did check the gas rings and they were fine. The bolt carrier key was not properly staked but it was tight.
Thanks for the help so far guys, keep it coming. 
First: Is his Bushmaster carrier Auto or Semi?
At first glance it sounds like his bolt has extractor problems. But that doesn’t really hold water if it’s working good in your 11.5" gun.
It sounds like a timing/dwell issue. I say this because it sorta acts like the bolt/carrier assembly is outrunning the bolt/carrier ability to extract and present a fresh cartridge at the right time. Things are happening much faster with the little 7.5" barrel.
I get the feeling that maybe the chamber pressure is too high at the moment of extraction and you are not getting a full extraction, the extractor is slipping off the cartridge rim then it’s just leaving the empty there on top of the mag as a new round is being presented for feeding.
If it was my upper I would try and narrow things down a bit (obviously). Take the the known good bolt assembly (yours) and install it in his Bushmaster carrier, fire it and tell us what happens. Or better yet try another BCG this time an AUTO carrier. Maybe a little more weight will help slow things down a bit…
Couple weeks ago a friend had a similar problem:
He assembled a used Colt 14.5" M4 barrel on a used Colt M4 flat top upper. He slides his know good semi auto no name BCG and away we go to our desert range. Functions perfect.
Since (like the OP) this BCG belongs in another rifle I says “hey let’s make sure it works with an auto carrier, so when you get the dough to buy a BCG for it we’ll know it will function”.
I slide out my know good Colt auto BCG from my 10.5" SBR and we install this into his “new” upper. No way in hell will it function. FTF & short stroking. We tried every known buffer combo including: carbine, “H”, H2, H3 even a Colt hydraulic buffer that I had from a Colt 9mm SMG. Nothing under the sun would work in that 14.5" Colt barrel except his semi auto '‘no name carrier’ and Colt bolt. Since he was happy just to get it running he let it go at that.
In my opinion I would have gotten to the bottom of that situation because I would not use it for anything other than a plinker. I would not trust that rifle for anything other that a noise maker. But it was not my gun so I left it at that. My best guess was: it was a problem barrel and that’s why he only paid $250.00 for an original 14.5" Colt barrel. Possibly the FSB was off and not delivering all the gas to the carrier, but I’ll never know for sure. Sorry for the ramble…
openbolt
I think it sounds like a combo problem. The bolt seems to have an extraction issue, which is exacerbated by the shorter 7.5" barrel. Even less time for chamber pressure to drop and brass to contract. I’d also try some heavier buffers maybe, to slow things down.
-RD62
Openbolt, BOTH carriers are semi auto carriers. The Bushmaster that he has is a standard one with a shorter bottom and my Colt carrier is an older cut away one so it has no bottom. The issue that concerns me most is that it runs better with the lighter carrier which kind of rules out the dewll/gas issue. It functioned fine with both the standard CAR buffer and the H buffer. I recommended he get a DD auto BCG but I don’t want to see him dump the cash for the bolt and it not work because it is too heavy. If I change out the extractor and spring and add a D ring it will cost less than $30.00 but the $159.00 for the BCG would hurt a bit if it didn’t fix the problem. I am going to swap bolts and use his carrier next and if it doesn’t work with my bolt and his carrier than it could be a gas flow issue with his bolt. I am also going to wait a while because he also ordered a Noveske can brake and that may also help with the issue.
THANKS EVERYONE!!! 
Corycop,
Just an FYI and I am sure that a few others will say the same. 7.5" uppers are notorious for being super picky. I have seen a few that would run good and some that ran like crap. Out of curiousity would this happen to be DPMS 7.5" build?
Gunz, the barrel and barrel only is DPMS. :rolleyes:It was the only one available that was chrome lined and in his price range. I figured that since it was a dedicated 7.5 barrel, not a cut down, it would be fine for his use (low round counts).
Cory,
Almost every 7.5" I have seen was made by DPMS. That would also include the barrel. None of them have worked reliably for any period of time.
Not a very promising post Gunz, I will try to do my best with this one. I’m always up for a challenge!
We are ordering a new BCG (DD auto) and KX3 brake tomorrow. We shall see what happens. Thanks for all the help guys!
I’d need to see and shoot it. I’m no fan of the 7.5" ARs from anyone.
In the meantime you can unload it and tie some 550 cord to the trigger guard and use the gun to keep your canoe from drifting…
Not quite the input i was looking for GotM4 :rolleyes: . As much as I love the boat anchor analogy, it’s not going to help too much. My son just asked why I was laughing at the computer. I tried to talk my buddy out of the short barrel but he was adamant about the 7.5" . I tried to talk him into the 10.5" but it was a no go. He was well aware of the cycling issues but he didn’t care. Thanks guys!
Cory,
Sorry you didn’t get the anwer you wanted. Maybe you could start by contacting DPMS and asking them what port size the barrel has. Then drop some more money on various buffers and ammo combinations until it’s right.
I would talk my friend out of it and talk him into a 10.5". A non-functioning weapon is essentially a bat. You can buy them at Wally World fairly cheap. Meiner zwei Pfennige.
Port size is .087 or .089 inch. can’t remember. I did get the gas tube with the barrel if that helps.