[Andrew Wyatt]

I have a mini'14 that's started ripping caseheads off of brass recently. I fixed the problem with a mark knifong reduced size gas port, but the rims are still visibly bent, and the extractor now tries to fly out of the top of the bolt from time to time. Up untill last year or so it was stone cold reliable.

What could the problem be?

[SDC]

My first guess is that there's a pit in the chamber, that's preventing the brass from being smoothly extracted. Have you got a couple of pictures of the brass?

[Grump]

We presume there has been no change in the rifle's diet, and that it does this with both reloads and fresh factory ammo? Does your experience include USGI M193?

One pesky possibilty is that the op rod is not going all the way forward but is still fully locking the bolt. This reduces the very useful "dwell time" between the bullet passing the port and the bolt unlocking and beginning extraction. Less time = higher residual chamber pressure = the brass does NOT want to leave it's warm, comfortable chamber yet. It will resist extraction like a teenager resisting taking out the garbage.:rolleyes:

The troublesome thing about this type of malf is that it is not readily detected. Feed, chamber, and fire are all unimpaired, because the bolt has rotated past the firing pin extraction. In an M1A, it is conceivable that similar conditions could leave the gas piston still in the cutoff position (or would recoil reset it???), but the Mini's fixed piston/moving cylinder design would, in my estimation, still give a full whack to the op rod.

[Andrew Wyatt]

We presume there has been no change in the rifle's diet, and that it does this with both reloads and fresh factory ammo? Does your experience include USGI M193?

my .223 can is mostly composed of "SOF test ammo", which is ammo that was confiscated from competetors for random velocity testing. Mixed headstamps, some if it is m193.

Recoil spring was replaced with a Wolf XP unit shortly after it starting ripping off caseheads. (which would tend to indicate against the oprod hypothesis, but i'm not ruling it out). I'll take it with me tomorrow and clean the chamber throughly again. next time i shoot it, i'll save the brass and take pics.

[Grump]

Recoil spring was replaced with a Wolf XP unit
If the spring rate at the very beginning of op rod movement is truly greater than on the unit you replaced, that certainly does point away from the dwell time theory--but only if the rod is not otherwise prevented from going forward.

Verrryy interesting problem, no? Awaiting further word...

[1911Tuner]

Sounds to me like a timing problem. The bolt is trying to move before the
chamber pressure has dropped to allow the brass to spring back and release its grip on the chamber walls.

The system is similar to the M-14's. Powder burn rate affects the
time/pressure curves. If the pressure spike is still on the high side when the port taps the gas off to actuate the op-rod, the op rod and bolt begin the extraction sequence while pressure is high, OR...too much gas volume
is getting to the piston too early or too quickly.

Since your ammo is most likely SAAMI spec, my guess is that possibly the
gas port in the barrel has eroded and enlarged, and is letting a larger volume of gas into the system than it was designed for.

I could be wrong, but I wouldn't think that it's related to the new spring.
In a Garand-type system, the action spring's main job is feeding a round and returning the rifle to battery, and buffering the shock of the bolt hitting the impact surface in the receiver. By the time the bolt starts to move to the rear, the pressure should have already dropped, no matter what the spring rate.

Keep us posted on what you find.

Tuner