headspace of rifle vs pistol
- Thread starter potatoe
- Start date
It has nothing to do with rifle or pistol. ALL CASES, pistol, rifle and shotgun, have datum lines.
It has to do with the design of the case. Straight, rimless cases, like the 30 Carbine, and most auto pistol cases (like the 9mm Luger and 45 auto) headspaces off the mouth - the datum line is on the mouth.
Bottle neck cases like most current rifle cases and a few pistol cases, headspace off the shoulder (the datum line is somewhere on the shoulder), like the 25 NAA, 30 Luger, 32 NAA.
The 357 Sig is the only exception that I can find - it is a bottle neck case, but headspaces off the mouth, cuz the shoulder is so whimpy, it is almost non-existent.
Because the shoulder has more support, and with high pressure rifle cases, you do NOT want the mouth of the case to be forced against the end of the chamber, because it can cause dangerous pressures by crimping the bullet at the chamber mouth, and not releasing it.
Straight pistol cases work at lower pressures, and do not suffer that problem.
swd
Silver $$ Contributor
What XBBR said^^^ The shoulder is large and strong and is an important dimension for case life. With a bottle neck cartridge if you headspaced of the oal cartrigde length then you would have a very complex headspace demision because you would also have to some how add the shoulder in the equation for support to make case fit but to run into case separation. The bottom line is the way we do it is the most practical way.
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And just to confuse the issue the third type would be belted magnums that headspace on the belt. Many magnum shoulders are well short of the chamber dimension until fired. At that point they can (and should) be treated like other bottleneck cartridges and have minimum sizing to fit the chamber better.
^^^^^ ++
The shoulders on new 264 WM cases in my 700 Sendero SF-II, blow forward 32 thousandths when fired - I treat fired belted cases like standard cases and adjust dies to headspace on the shoulder.
As in .032"? I'd have that corrected soon. Stretching those cases that much once is enough to weaken them.
"As in .032"?" Yes...
The cases don't stretch - the belt restrains the case in position for the first firing... when reloaded, the shoulder positions the case thereafter.
No case stretch (I measured).
So, the shoulders moved forward .032" and there was no case stretch involved? You need to think about that!
Belted cases use the belt for headspace control - no matter how deep the chamber is, the case cannot move forward, because the belt retains it in place.
As it can be seen - the case are "locked" in place by the belt, against the shelf in the chamber. Even if the case shoulders were back 100 thou (0.100"), they would be blown forward, but the case would not stretch (more the the SAAMI difference in the belt min/max ~ 7 thou-ish)
It is no different than when you are forming a 25 Krag, where the case is held in place by the rim, and the shoulders are blown forward (a whoppin' hellova lot.
Belts and rims do the same job.
Once the cases are fired the first time, you set the dies to just kiss the "new" shoulder...
Lookie.. pictures
[note]... I have owned this rifle since 2006, and I have never had a problem with case stretch)
OK. You win!
butchlambert
Site $$ Sponsor
I'm not sure I buy no case stretch. Think about it. .032!!
A case stretches when there is "head" - "space" (the space caused because the cartridge is shorter than the chamber)... the primer forces the loaded case forward, leaving all that real air space in the back.
If the chamber is wet (with oil, or bore cleaner) the case walls (body) cannot grab hold of the chamber walls, so the pressure pushes the whole case back to the bolt face, and completely fills the chamber (a good thing on a first firing... Jack Neary agrees with this).
But if the case walls (the body) and chamber walls are dry and there is empty space... "head" - "space", the forward case walls grab the chamber and sticks to it (the Blish effect), and now the case stretches, just in front of the web... the case now still completely fills the chamber but it is now longer by the same amount the it stretched at the web. The walls just in front of the web are thinned, and the case is on the way to a premature death after one firing.
If you have a rim, or a belt, those will prevent the case from being pushed forwards (more then than the SAAMI min/max, a few thou), no matter how much space there is in front of the cartridge If the shoulder on a rimed or belted cartridge is 0.032" shorter than the chamber (or 0.100" for that matter)... the case head is held solidly in place against the bolt, and the whole shoulder is blown forward... think 6mmBRX - the whole shoulder is blown forward 0.10" (full tenth of an inch)... but instead of a belt or rim, long seating is used.
You young pups probable don't remember cartridges like the 25 Krag, and the whole Gibbs line of cartridges, but it was common to cut chambers WAY longer than the existing shoulder and blow the whole damn shoulder way forward - with no problems or case stretch. Just like Ackleys now, EVERYBODY had a Gibbs back in the day.
Once the shoulder is blown forward to fill the chamber, then you set the FL or Bump die to just kiss the fired shoulder... and the cases do not stretch, ever.
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butchlambert
Site $$ Sponsor
Who is Jack Nealy? Quote: Jack Nealey agrees with this.
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