problem with 7.5 upper

[TonyT]

I picked up a 7.5 "kitty cat" upper for my SBR.

thing is, I can't get it to function properly.

insert mag, drop bolt, chambers a round fine. fire. doesn't extract spent casing, and tries to feed a fresh round in the still occupied chamber.

I can manually cycle it, by pulling the charging handle to the rear. feeds just fine.


wondering what could be the issue. I'm thinking maybe I need to try a lighter buffer spring?

[bkelm18]

Probably a different weight buffer is in order.

[TonyT]

would it be a lighter buffer?

[Rightwinger]

I would check for a rounded extractor claw. Is it marking your brass at all? Should be a slight mark where it slid over the rim and locked on. Take your bolt apart and look for a smushed extactor spring or if its not got a spacer rubber in it.

[mw.]

You might look at the extractor and spring. Check the chamber for rough spots. How are you getting the brass out?

[Rightwinger]

Dang MW GMTA

[Reservoir Dog]

Short barreled uppers can be picky, especially 7.5" uppers. They take a bit of punishment. There are a few things you could do to try and fix the problem.

First: I would do an extractor upgrade on your bolt. You can buy the O-ring upgrade kits from several online companies. I would suggest you try Bravo Company.

Second: What weight buffer do you have? If you are running a standard weight buffer, I would suggest you go with a heavier H or H2 buffer. The heavier buffer will slow down the cycling, and allow some of the gas pressures to subside before extraction begins. Switching to a F/A bolt carrier will also accomplish the same thing, but switching buffers is much cheaper. :D

I am betting the O-ring extractor upgrade will solve the problem, but a heavier buffer may also be needed.

[TonyT]

thanks for the info guys.


the BCG works fine in longer uppers. I'll look for that upgrade stuff for the BCG. standard weight buffer I"m pretty sure.

[Reservoir Dog]

Ooops! I meant to post a link to the extractor upgrade, but forgot.

Here it is: http://www.bravocompanyusa.com/BCM-Extractor-Spring-Uprade-Kit-p/bcm%20extractor%20spring%20%203pack.htm

[Mugster]

If you bought the upper complete, I'd send it back to wherever it came from and get them to fix it. It should run right out of the box. If it doesn't, somebody screwed up.

If its a home build, I think I'd go with a smaller diameter gas tube. You can fork around with the extracter, but those o ring kits don't last 2000 shots.

[TonyT]

picked it up used.

[tngunman]

I had one, it needed a pig tail gas tube and a brand new bcg to function correctly. I ended up selling it because I did not trust it 100%.

[LA_357SIG]

I had a 7.5" pistol kit from Model1sales and it ran just fine. It came with a pistol specific buffer tube and buffer. I'm not sure about the spring itself but I don't think it is a carbine assembly.