AR help

[REDdawn6]

A friend needs some help. I was supposed to meet him and help him strip and clean but didn't happen. Life. He went shooting and said it started jamming after 70rounds.
His texts. No. And I took it out and after about 70 rounds it started to jam after every third round. And I would have to clear it. My dad was going to show me but he's been working so I still haven't cleaned it
No pictures. I would pull the trigger and nothing would happen so I would clear it and it would fly out. And I could shoot another 3 rounds before it would happen again. The dimple on the back of the shell was a small dent compared to the shells that were shot.
I'm OK at diagnosing stuff but you guys are much better. Thanks guys.

[milotrain]

trigger spring might be backwards.

[REDdawn6]

I'm thinking maybe bad or incorrect hammer/triggers springs. Or maybe dirty or bad firing pin. He said it's a new Bushmaster.

[LeadFarmer74]

What ammo were you using?

[Calico1404]

What ammo? Tula with a super wet new bolt can cause a lot of build up in your chamber.

[milotrain]

I've seen new ARs with the spring backwards. I've never had a dirty/dry gun do the three shoot to one fail trick. It usually does the shooty shooty shooty fail fail fail fail fail fail shooty fail fail fail fail fail fail.

Firing pins are cheap but it'd have to be way out of spec to only work sometimes. How light are the failed primer strikes? Is it just floating pin strike, aka could the hammer not be fully cocked or even falling on the primer in the failed cases?

[REDdawn6]

He did not say what Ammo. I can ask. He did say if he shot faster it would fire rounds but anything else s
, slower, it would not fire.

[boludo12]

I had a similar problem after assembling my rifle wrong after breakdown and cleaning. One of the small springs "sprung" away and I didnt see it. Assembled without spring and kept getting light dimples or double shots.

trigger spring might be backwards.

[REDdawn6]

All good suggestions. Thank you

[REDdawn6]

Oh man.. At work.. Wrong section too.