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Discussion Starter · #1 ·
How I Learned to Stop Worrying and Not Make a NASA Level Project About Reloading Ammunition.

There once was a guy that used to shoot in NRA High-Power Rifle Matches, he didn't win any, never even came close, but he did managed not embarrass himself with his scores. He reloaded his own ammunition when it was allowed. He set-up the resizing die the way it is described in the instructions (he was weird that way, reading instructions) by screwing in the die until it touched the shell holder and adding 1/8 turn. Bench-rest testing of his reloads always showed they were pretty good, accuracy wise. He never bothered with measuring the length of fired cases and fiddling with the dies to length size cases to a few thousandths less. He saved all that money he would have spend on the measuring equipment and bought himself used Creedmoor spotting scope stand.

(All of the following applies only to rimless bottle neck cartridges.)

When you measure the headspace in a rifle, you are actually measuring the distance from the where the locking lugs seat in the receiver (or barrel extension) to the reference diameter on the shoulder, "A" in the drawing. Since you know the length of the gauge you infer that the distance from the locking shoulder to the face of the bolt is within specified limits. Because the rifle is a mechanical device made on a production line and required to work with minimal friction, there is some clearance around the locking lug(s), dimension "B". This is known as "bolt over-travel." In the M1/M14 this is around 0.010" to 0.015", the AR system around 0.009" to 0.013".


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In a semi-automatic (full automatic too), when the bolt goes forward, there is a lot of mass moving and thus a lot of energy to be dissipated, more that the thin brass of the shoulder can handle, so what stops the forward motion of the bolt is when the front of the locking lug hits the front of the locking well*. This will ram the case further into the chamber and bump the shoulder back the necessary amount.

Let's assume you have an M14 with a headspace of 1.6360", a bolt over-travel of 0.010", and a cartridge with a SAAMI minimum case length of 1.6270". when the round is chambered the bolt moves passed the 1.6360" point and stops when it runs into the front of the lug well. This leaves only 1.6260" left for the cartridge case, and it gets crushed 0.001".

When the cartridges is fired, the case does not move until the bullet uncorks from the case, since there are no external forces on the cartridge. In the case of the M1/M14, FAL, and AR, the hammer will keep the bolt in the forward position. Once the bullet uncorks, the pressure in the case pushed both forward and aft. The forward push moves the bullet forward and pins the shoulder of the case to the shoulder in the chamber. The pressure pushing rearward moves the rear of the case backward and with it the bolt until the locking lugs make contact and pick up the load. This stretches the case to theoretically to the chamber headspace. Usually there is residual pressure (a few hundred psi) in the chamber when extraction starts keeping the shoulder pinned in place when the bolt opens stretching the case a little more.

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So, basically, the case will always stretch at least the amount of the bolt over-travel plus the extraction stretch, no matter where you bump the shoulder to. In the example above, if we set the case length to 1.634" instead of 1.6270", the crush would have been 0.008" and would have stretched the exact same amount.

Can you minimize bolt over-travel? You could select a bolt that has the largest lugs and a receiver with the smallest locking gap, but generally no, you are at the mercy of production. You can, however, increase it. And many people do, albeit without knowing so. Lapping bolts removes material from the back of the bolt and front of the receiver locking abutment, effectively moving the bolt to the rear and increasing the bolt over-travel ("B" below). If you lap just to get a good contact patch you probably are only removing 0.0005" to 0.0010", so no big deal, just don't go crazy.

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Bolt actions are different. The forward motion of the bolt is rarely as violent and the extractor engages the rim during feeding, so there is less, if any crush due to over travel.


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* In an M1/M14, in a FAL or AR it stops when the front of the bolt itself hits the rear of the barrel.
 

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Discussion Starter · #2 ·
Also, this the important reason when chambering the first round in a rapid fire string you should allow the bolt to fly home. The case crush will change the initial case volume, which can have a big impact on velocity.
 

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There once was a guy that used to shoot in NRA High-Power Rifle Matches, he didn't win any, never even came close, but he did manage to not embarrass himself with his scores. He reloaded his own ammunition when it was allowed. He was shooting full bore 1000 yd. loads. He needed reading glasses (which he was not wearing) to see things up close and picked up all brass that landed where his was "supposed to be", without reading any case head information. From there it went right to the prep and loading processes with nary a second glance at the condition of the cases or the headstamp information. He did, however, manage to embarrass himself with all of the crazy things that happened to his rifles and ammunition in matches. Truth is stranger than fiction.

Danny

Sent from my SM-N950U using Tapatalk
 

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What does longer dwell have to do with this and brass life?
 

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Discussion Starter · #5 ·
What does longer dwell have to do with this and brass life?
Generally speaking, the longer the "dwell time", the time from primer ignition to bolt unlock, despite what the AR community thinks it means, the lower the residual chamber pressure when extraction starts in earnest. This tends to reduce additional stretch during extraction.
 

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lysander, along with the dwell time , will shutting off the gas system and taking a shoulder set back measurement work the same as what you have presented in the above with the bolt slamming forward to set the shoulder back, thus compressing the total volume. It's just a thought for a winter project, using your drawings and set of gauges to get a complete chamber an shoulder set back test.. Or just over thinking it LOL...
Carry On !!
 

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Discussion Starter · #7 ·
I seem to recall someone fired a few round with the gas cut-off and a few round in regular fashion and measured the case length afterwards. I don't recall what his results were.
 

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So if I lap my bolts in and I cut my chambers to 1.630 measured with a PTG HS gauge what would be the over travel? I set my case HS typically 1.627 my fired cases typically are 1.632. Yup I am that guy that has a redding instant indicator and knows exactly what my case HS is. As easy as installing a die and about 30 seconds to calibrate.
 

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lysander, along with the dwell time , will shutting off the gas system and taking a shoulder set back measurement work the same as what you have presented in the above with the bolt slamming forward to set the shoulder back, thus compressing the total volume. It's just a thought for a winter project, using your drawings and set of gauges to get a complete chamber an shoulder set back test.. Or just over thinking it LOL...
Carry On !!
I’ve used a comparator to check with gas on and off and found no difference. Each case was checked in a Sheridan case gauge before hand. Asked this question here a few years back and never did get a satisfactory answer. Headspace is 1.6315 according to tag on it from Springfield when I bought it in 91. If I were to bump the shoulder back.004 after firing it wouldn’t be close to sammi specs.
 

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Thanks for the expert explanation of what I had long suspected occurs in semi automatic operation. My rough “shade tree mechanic” measurements on the right lug with feeler gauges seems to have .010” if I recall, which may or may not be correct.

Regardless, I knew bolt velocity probably would crush the case forward despite trying to maintain that mystical .004” shoulder bump.
 

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Discussion Starter · #11 ·
So if I lap my bolts in and I cut my chambers to 1.630 measured with a PTG HS gauge what would be the over travel? I set my case HS typically 1.627 my fired cases typically are 1.632. Yup I am that guy that has a redding instant indicator and knows exactly what my case HS is. As easy as installing a die and about 30 seconds to calibrate.
On an M1/M14 you can measure the over-travel with a feeler gauge, AR, not so easy.

Close the bolt and insert feeler gauges behind the bolt lug until you can't.
 

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Gotta wonder about this over travel. Upon chambering, the ejector pushes the case forward and bolt backwards to the extent that the extractor will permit. As the Op rod stops on the piston the Op rod does not push the bolt forwards once bolt closed. The garand is a bit different as the Op rod can push the bolt forward
 

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On an M1/M14 you can measure the over-travel with a feeler gauge, AR, not so easy.

Close the bolt and insert feeler gauges behind the bolt lug until you can't.
I will do this but I am pretty sure there is none. This is how I built them. Is there supposed to be over travel?
 

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I will do this but I am pretty sure there is none. This is how I built them. Is there supposed to be over travel?
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The over-travel is mostly when the chamber is empty. If there is no over-travel with an empty chamber, then the forward part of the bolt is probably in hard contact with the end of the barrel, and the locking lugs have 'just enough' clearance to rotate closed.
 
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