I have purchased a radical firearms upper and after 3 uppers, we're having problems cycling the rifle on full auto properly. If I use a 11.5+ upper, the rifle works just fine. Once we go down to a 10.5 inch, I'm getting ton of FTFs with it. there appear to be light primer strikes on the primers and the round doesn't go off. Not sure if the sear resets or not.

I have a carbine? (3 oz) buffer in there right now.

ironically, when I use a colt BCG (not theirs, which look dimensionally the same) I get very few to no FTFs. They sent me two BCGs (one of them being nickel boron) and same results... Before they sent me the 3rd upper, they shot 3 mags through it on a HK 416 post ban lower and it shot fine.... Comes to me, and it's fine for the 1st 10-15 rounds, then continous FTFs one after another...

we're thinking it's possibly weak springs in the trigger....

so looking on suggestions on what spring to get to beefen up the hammer. I don't want the weight of the trigger any less than milspec (duty rifle). Just looking to make the hammer hit harder as we think the spring is just getting worn out.

It works fine in semi. only FTFs in full auto...

http://youtu.be/CuU1As1vj0M

We think it's the lower, so any suggestions will be appreciated. Only thing it can be is the hammer spring in my opinion. It's just odd that it works fine on a full size upper.

I can't load your video but one cause of FTF is over gassing, which seems to be common on 10.5 uppers*, the speed of the BCG is to much for the magazine spring to keep up. a stronger spring (WOLFF XP or Sprinco red) as well as a H2 or H3 will solve this issue.

* I've watched 4 videos in the last week with 10.5 uppers ejecting damn near 12:00(suppressed and/or full auto).

The magazine not keeping up is not the problem in this case as the cartridge is getting chambered properly. This is likely "bolt bounce" since it is occurring in full auto. A heavier buffer will likely fix the problem by decreasing bolt bounce and slowing cyclic rate. Search for "bolt bounce" on you tube and see what is happening in slow motion. The bolt is bouncing out of battery as the hammer is falling, preventing the firing pin from fully extending and dimpling the primer enough to set it off.

Thanks guys. I do have a H buffer (3.8 vs 3 oz) I wonder if that will help...

Try it and see. If not them go to H2 or H3. Generally use the lightest buffer that fixes the issue.

That always sucks because that means I have to buy several buffers to see which works, and shooting all this ammo gets expensive. I've shot over 1000 rounds trying to figure out what's wrong with this damn thing.

I'm just so baffled why it works better with the Colt BCG... I think they are the same weight... (They both weigh 11.5 oz)

I just ordered a GI H2 buffer from brownells... now if I get a Wolff XP spring on top of that, is that going to be too much? as in too much resistance causing short strokes now?

I have no idea how many rounds that spring has. The rifles were gov surplus, (but from USAF as GUU-5P) so no idea on round count. I have one of the better looking ones, which could indicate not as used.

An H2 and a Wolff XP will not by any means be too much. I run the Spikes ST-T2 and Wolff XP on my 10.3" and its perfect. I may even go to an H3 or ST-T3. A carbine buffer and standard spring is far too light to be ideal for your setup.

Send an email Alan at Sprinco see what they recommend...

[email protected]

http://www.sprinco.com/tactical.html

If you have a H buffer, then you only need to purchase a H2 buffer to have all the internals needed from each buffer to make whatever buffer weight is needed. Or, order a H3, and you can upgrade that carbine (3.0 oz) buffer to a H buffer, and the rest of the components can be used to swap with the H you have already. As has been mentioned, you may be experiencing what is referred to as bolt-carrier rebound (bolt "bounce"). The carrier hits the barrel extension as the bolt goes into barrel, but bounces back enough that the hammer can still fall, but is not able to strike the primer.

Carbine (3.0 oz) — 3 steel weights; 3 rubber pads
H(1)  (3.8 oz) — 1 tungsten & 2 steel weights; 3 rubber pads
H2 — (4.6 oz) 2 tungsten & 1 steel weights; 3 rubber pads
H3 — (5.4 oz) 3 tungsten weights; 3 rubber pads

You can also contact Alan by phone; and he will take the time to help you out, answering any questions you may have that he is able to give an answer for. (512)-926-3100

guys I ordered a white sprinco and a H2 buffer... all is well now. I guess the issue was indeed bolt bounce. my spring seemed fine however, but it doesn't hurt to have a new spring in there. plus with this chrome moly (or w/e its called) coating the sprinco has, I don't hear the buffer tube to spring sound that is very annoying (at least to me)

the cyclic rate did go down a little, but that's the least of my worries. I was more worried about a properly operating firearm.

Topic Moved

Glad it worked out! Alan has helped me fine tune several builds and his springs will out live most of the other parts of your gun.  Some say it's a waste of money, but how many carbine springs will a high volume shooter go through?

Mark LaRue uses them as OEM on at least some of his rifles and he's no slouch