Importance of case length?

P

Prof Young

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Loaders:

It occurred to me today that somewhere along the way to becoming a long time reloader I got the idea that case length, for a necked down rifle cartridge, is not as critical if you are shooting a bolt action gun but more critical if you are shooting a semi-auto.

I'm now thinking this idea is incorrect.

Talk to me, please.

Life is good.

Prof Young
 
Case length (head to mouth) is critical for both action types. If too long, force from firing pin impact can set the case shoulder back enough to let the case mouth be driven into the tapered chamber mouth "crimping" the case mouth into the bullet raising pressure and reducing accuracy. Case length should be at least. 010" less than chamber length.

Case headspace (head to shoulder reference) needs to be at least. 001" shorter than chamber headspace on all action types. No more than. 002" shorter is ideal for maximum case reloading life and accuracy.
 
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I used to think that rifle cartridge case length was really important. But, while trimming cases and watching the Super Bowl many years ago, I badly over trimmed my 220 Swift cases, to the tune of maybe 1/8 inch. Those were all the cases I had, and new ones were very tough to get back then. Happily, the Swift cases have long necks, so I just loaded them up and hoped for the best. They shot great, so I used them till they wore out, and that was many loadings since I was Partial Resizing back then.
 
Well I guess that a bolt action might tell you its a problem and a semi auto would just go boom when the firing pin went down and it was not fully closed.

I now over-trim to ensure that the cases don't grow past the max before I anneal. My cases are plenty long.

But if you are talking Full Length dies vs partial resizing then ........
 
Exceeding the max length is bad, as described. If the shoulder is in the right place, and the neck is a few thousandths too short, the rifle likely won't notice and neither will you, unless you measure it.

However, if you are crimping, different case neck lengths will matter.
 
RC20 said:
Well I guess that a bolt action might tell you its a problem and a semi auto would just go boom when the firing pin went down and it was not fully closed.
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Which semiautomatic rifles allow their firing pin to touch the chambered round's primer before the bolt is closed in battery?

I'm not aware of any semiautomatic rifle whose firing pin is heavy enough nor fast enough to dent a primer at least 20 thousandths (which is the typical minimum to fire rifle primers) after the bolt is in battery.
 
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Ya that seems like it would be very dangerous, if a firing pin could extend before bolt fully closed, the gun would have to have a defect, I think I have heard that maybe a firing pin could move forward a bit with the inertia of the bolt slamming shut, but im thinking that would just be a slam fire, not sure?
 
Bart: Slam fires come to mind and I would assume (could be wrong) if its not fully close and you pull the trigger?

How many people look to best sure the semi auto is a fully close bolt?

I do check the K-31 bole rotation to closed each and every time (semi auto in its bolt type but not the actual mechanism which is yours truly in this case)
 
RC20, don't know how many people verify firing pin function and fully closed bolts

M1and M14 rifles whose sear engagement is honed to minimum plus and trigger backlash reduced too much will "double" (2 rounds fired with 1 intentional trigger pull) when the rifle moves forward from the first shot's recoil. That pushes the trigger forward against the trigger finger enough to fire the next round as the bolt closes into battery on the newly chambered live round.

Seen this happen twice; shooter's claimed a slam fire from poor firing pin fit or sensitive primer but both rifles fixed by increasing sear engagement and trigger backlash. And increasing trigger pull up to at least 4.5 pound spec.
 
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My Anderson AR15 started getting quite a few slam fires, I couldn't figure it out, it was always 2 rounds, no more, I called Velocity triggers tech support, and they told me that the trigger has 4 set screws holding it in tight. Two on each side, they said take the top 2 screws out, which basically hold in the bottom 2, and tighten the bottom screws, then put in the top screws tightly and my problem disappeared. It was a trigger that had just slight movement, when it should have been solid.
 
Bart B, as a novice handloader and new M1D owner who is consider reloading 30-06, i read you comments with great interest. While I wasn't thinking about monkeying with the trigger/sear, I am now warned about potential consequences of doing so. I am also getting educated about case length ramifications. Thank you.
 
The bolt on a STEN has a permanent firing pin machined right into the face and would detonate a partially chambered round if the brass was too long. I'm not sure but I'd guess any gun that fires from an open bolt design could do the same.
 
Lexspeed said:
Bart B, as a novice handloader and new M1D owner who is consider reloading 30-06, i read you comments with great interest. While I wasn't thinking about monkeying with the trigger/sear, I am now warned about potential consequences of doing so. I am also getting educated about case length ramifications. Thank you.
Click to expand...
You're welcome.
 
Lets clarify a couple things, two DIFFERENT causes resulting in firing are being called "slam fires", and while the result is the same (the round fires) they happen for different reasons and should not be confused.

Slam Fire refers to the closing bolt firing the round, whether there is a finger of the trigger, OR NOT.

Several common military rifle designs use a floating firing pin, which, literally bounces off the primer when the bolt closes. This is intentional, and why a certain hardness of primer cup metal is required for proper, safe operation.

Things are engineered so that the round won't be fired, until the hammer falls, but malfunctions. such as a stuck firing pin or using the wrong primer in the ammo can result in the round firing unintentionally. This is a slam fire.

A rifle doubling because of recoil jarring off a too light engagement of the trigger/sear/hammer is not a slam fire in the same sense.

Old vets can tell you that there are some people who can get an M1 or M14 to double, or even more (with a stock trigger group), by holding the trigger and the rifle in just the right way during recoil, and that is a bump fire. Done simply by technique not using any special (now banned) stock. I could never learn the trick to be able to do it intentionally but I've seen it done, accidentally.

it is entirely possible to create a rifle where the shock of just the action cycling shut will jar off the sear engagement, firing the gun. Not a safe condition, it is a malfunction that needs immediate repair. This usually happens when people try to get the "best possible" (meaning lightest) trigger pull. I've even done it with a bolt action, just by adjusting the trigger TOO much.

SO, if the rifle fires as the action shuts, with no finger on the trigger it's a slam fire and there a couple different causes. If it fires a second time while you're pulling the trigger its most likely either being jarred off, or you have unintentionally bump fired it.

A severely overlong case, one that doesn't allow the rifle to fully go into battery (and with a hard enough primer for a floating firing pin system) shouldn't slam fire, unless its in a fixed firing pin gun, such as many SMGs.
 
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