FTF fix?

Moderators: e292644, blueeyedwolf

Post Reply
User avatar
friedchickenwaffles
II
Posts: 6
Joined: Wed Aug 15, 2018 3:50 pm
Country: USA
Location: PA

Sat Sep 15, 2018 4:42 am

I've got an issue with loading the first round off a topped off magazine where the bullet tip just rams into the receiver just under the feed ramps. Unsure why there are no extensions in these uppers. What's the risk if I were to lightly dremel/file an extension in?

Fwiw this issue began after swapping a POF anti tilt buffer tube and a geiselle mg42 style spring and h1 buffer in.


User avatar
Arkane
C
Posts: 150
Joined: Tue Jun 20, 2017 12:35 pm

Sat Sep 15, 2018 4:31 pm

Did you switch back to a stock buffer/spring and see if that fixes it?
User avatar
friedchickenwaffles
II
Posts: 6
Joined: Wed Aug 15, 2018 3:50 pm
Country: USA
Location: PA

Tue Sep 18, 2018 1:36 pm

Arkane wrote:
Sat Sep 15, 2018 4:31 pm
Did you switch back to a stock buffer/spring and see if that fixes it?
No, but I can't see how the spring or buffer would be the issue. My guess is it's the tube, which will be a bit disappointing if true. I'm out of town right now but I'll probably start mixing and matching buffer tube/spring/buffer combos to see what works or doesn't. I want that anti-tilt tube to work though.
User avatar
e292644
Site Admin
Posts: 420
Joined: Thu Apr 20, 2017 5:47 pm
Country: USA
Location: AZ
Contact:

Tue Sep 18, 2018 5:03 pm

Safety first!
How did you release the bolt? By pressing the bolt release?
Have you try to pull the charging handle then release the bolt slowly see how it load the ammo?
Watch carefully how the bolt move forward and if there's any tilting movement.
User avatar
Arkane
C
Posts: 150
Joined: Tue Jun 20, 2017 12:35 pm

Wed Sep 19, 2018 10:17 am

friedchickenwaffles wrote:
Tue Sep 18, 2018 1:36 pm
Arkane wrote:
Sat Sep 15, 2018 4:31 pm
Did you switch back to a stock buffer/spring and see if that fixes it?
No, but I can't see how the spring or buffer would be the issue. My guess is it's the tube, which will be a bit disappointing if true. I'm out of town right now but I'll probably start mixing and matching buffer tube/spring/buffer combos to see what works or doesn't. I want that anti-tilt tube to work though.
The energy the BCG mass has traveling forward greatly impacts the ability to strip rounds from a mag, and if the spring is either too strong or weak it effects the distance & speed the BCG travels at. If it moves freely within the tube then the tube shouldn't be an issue. Before I spent any money or started permanently altering anything I'd swap the stock spring back & buffer in and try it, and if that works then swap int he H1, then the spring, etc.
User avatar
changhsu08
II
Posts: 17
Joined: Sat Jun 03, 2017 10:24 am

Mon Sep 24, 2018 9:53 am

I had exactly the same problem. Switched to the anti-tilt buffer and extra power buffer spring combo, then problem solved.
User avatar
friedchickenwaffles
II
Posts: 6
Joined: Wed Aug 15, 2018 3:50 pm
Country: USA
Location: PA

Thu Oct 04, 2018 6:00 pm

To me, it seems the obvious issue is the lack of m4 feed ramps and extensions. The bullet is basically ramming into the flat face of the upper below the barrel extension. If there was a ramp it would be a non issue.

Running the bolt slowly by hand is almost a guaranteed failure. If I use the bolt release, or let the charging handle fly, it feeds most of the time.
User avatar
Arkane
C
Posts: 150
Joined: Tue Jun 20, 2017 12:35 pm

Thu Oct 04, 2018 10:15 pm

friedchickenwaffles wrote:
Thu Oct 04, 2018 6:00 pm
To me, it seems the obvious issue is the lack of m4 feed ramps and extensions. The bullet is basically ramming into the flat face of the upper below the barrel extension. If there was a ramp it would be a non issue.

Running the bolt slowly by hand is almost a guaranteed failure. If I use the bolt release, or let the charging handle fly, it feeds most of the time.
Which is exactly an illustration of how the speed and mass of the BCG effects feeding. The T91 is an evolution of the A1 design. These designs typically run flawlessly if the bolt has the proper speed and mass when it strips the round from the mag. Changing too much in the mass or compression rate of the recoil chain (BCG/buffer/spring) is going to affect cycling mass and feeding.

Running the stock setup if you have problems feeding consistently the next place to look would be the mag and lower, but since it reliably strips a round when released from lock during dry fire indicates you have taken the recoil chain out of tolerance by replacing some of those parts. During the stripping and feeding steps of the cycle of operation if the BCG doesn't have enough energy on return (due to either to stiff a spring, too heavy a buffer, or too light a BCG) you'll run into feeding issues.

I stand by my assessment that the Geiselle spring & buffer is the primary culprit. Have you swapped the original back in and see if it ran as suggested?
User avatar
friedchickenwaffles
II
Posts: 6
Joined: Wed Aug 15, 2018 3:50 pm
Country: USA
Location: PA

Tue Oct 09, 2018 4:49 pm

Arkane wrote:
Thu Oct 04, 2018 10:15 pm


Running the stock setup if you have problems feeding consistently the next place to look would be the mag and lower, but since it reliably strips a round when released from lock during dry fire indicates you have taken the recoil chain out of tolerance by replacing some of those parts. During the stripping and feeding steps of the cycle of operation if the BCG doesn't have enough energy on return (due to either to stiff a spring, too heavy a buffer, or too light a BCG) you'll run into feeding issues.

I stand by my assessment that the Geiselle spring & buffer is the primary culprit. Have you swapped the original back in and see if it ran as suggested?
I haven't, but I haven't had time to put rounds through it in the current config either to see how it cycles under fire. I'll bring the other spring along when I do, and if I get issues I'll swap it in. The only other buffer I have is an h1 as well, so changing that isn't going to make a difference.

I did notice though that the issue basically disappeared when I switched to an aluminum gi mag. The mag that I had in when the problem showed up was a Sig (rebranded IMI) polymer. They work fine in my other rifle, but I suppose it could well be the whole problem here.

I appreciate the lesson on gun science though, thank you!
Post Reply