AR like straight pull bolt action - case extraction?

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diegocn

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On a traditional bolt action rifle, the bolt lug is sloped, hence rotating the bolt to unlock the action also extracts the case by a tiny amount. This extraction force is very strong due to the leverage provided by the slope (called the primary extraction).

From what I understand, the AR15 breech locking interface does not provide any primary extraction, as the bolt lug is square. Would this be a problem in MRA/Specter's design? Normally this shouldn't be an issue, but when firing a particular hot load, there would be the risk of bolt hard to pull back? Have they done something to address this issue?

I guess there's always the rack charging handle against table fix.
 
SBI gave a report on how various ammo worked and also how some barrels worked better than others.
From my experience with a bolt-action Zastava rifle in 7.62X39, lacquered steel cases were much harder to extract than the copper-washed ammo.
 
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diegocn said:
On a traditional bolt action rifle, the bolt lug is sloped, hence rotating the bolt to unlock the action also extracts the case by a tiny amount. This extraction force is very strong due to the leverage provided by the slope (called the primary extraction).

From what I understand, the AR15 breech locking interface does not provide any primary extraction, as the bolt lug is square. Would this be a problem in MRA/Specter's design? Normally this shouldn't be an issue, but when firing a particular hot load, there would be the risk of bolt hard to pull back? Have they done something to address this issue?

I guess there's always the rack charging handle against table fix.
Click to expand...

You are correct, AR bolt lugs are cut at 90 degrees as are the lugs in the barrel extension, therefore there is no primary extraction. This can make extraction difficult at the best of time, and nearly impossible when things are complicated by over pressure, dirty or lacquered casings.
We made a number of manual action ARs for a company in the UK years ago and test firing them found that extracting spent casings required much more effort than a normal bolt action does, for this reason.
On an AR style manually operated action a steel charge handle is your best bet, the standard aluminum ones don't tend to last long normally.
 
diegocn said:
On a traditional bolt action rifle, the bolt lug is sloped, hence rotating the bolt to unlock the action also extracts the case by a tiny amount. This extraction force is very strong due to the leverage provided by the slope (called the primary extraction).

From what I understand, the AR15 breech locking interface does not provide any primary extraction, as the bolt lug is square. Would this be a problem in MRA/Specter's design? Normally this shouldn't be an issue, but when firing a particular hot load, there would be the risk of bolt hard to pull back? Have they done something to address this issue?

I guess there's always the rack charging handle against table fix.
Click to expand...

I can not speak about the AR designs, but I know on Mauser's and similar, that about first 50% of bolt opening effort is on flat faces - no extraction occurring at all until past about 50% of bolt handle movement. You will find on Lee Enfield's, P14 and M1917, that their helical breeching results in the bolt moving rearward almost immediately as the bolt handle is lifted, or at least the locking lugs within the receiver allow the bolt to move rearward - the actual extraction cams seem to engage much further up in the bolt opening motion, but the bolt face "pressure" on a tight cartridge comes off almost with very first bolt handle motion. So, the helical breeching certainly seems to have continuous "push" to chamber a dirty or bent cartridge, whereas the Mauser and similar seem to get all their forward motion, and conversely, all their extraction, done in less than the "top" half of bolt handle motion downwards.
 
The AR's bolt twists a bit as it's pushed into and pulled out of the front of the bolt carrier, due to the cam slot where they join, and the carrier does not rotate as it cycles, so the wee bolt at the front is apparently turning and doing primary extraction and locking in/out of the barrel extension as it's pushed into place and then as the carrier cycles back due to gas operation or the charging handle.
 
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