Us Springfield Armory M1903 help

[Jim Watson]

Well, the headspace of a rimmed round is the thickness of the rim/depth of the rim recess in bolt or barrel. But shoulder setback would still matter.
I would expect, that if rimmed cartridges were more "modern" and common, that they would be treated like belted magnums; sized so that the case shoulder was located at the chamber shoulder.

The late Remington Lees had front locking lugs.

 

[James K]

Headspace in a rimmed case is the same as for a rimless case - the distance from the stopping point of the case to the face of the breechblock.

The difference is that for a rimmed case, the stopping point is the face of the breech, where for a rimless case it is a point in the shoulder of the chamber.

In a rimmed case, headspace is some defined distance greater than the rim thickness. For example, the .30-30 rim thickness spec is .062"-063"; the headspace spec is .063"-.070".

Belted magnums really are rimmed cases. When H&H came up with the .375 H&H Magnum, they wanted to use a rimless case for easy feeding. But they found out that the tiny shoulder would not be enough for longitudinal case support against the firing pin. Without that support, misfires and inaccuracy (due to inconsistent primer ignition) would result.*

So they built in what amounts to a new rim, the belt. Contrary to common belief, the belt does not add strength to the case; it is too far back to support the case walls.

If the shoulder of a rimmed or belted case is set back during re-sizing, firing in a rifle with proper headspace will simply blow it back out to fill the chamber. Only if the rear of the case can move back far enough to exceed the elasticity of the case material, will the case separate. That will not happen if the headspace is within specified limits, since those limits take the elasticity of the case into consideration.

There is a belief that with a case like the .303 British or .30-40 that there is some need for the case shoulder to be tight against the chamber shoulder, and that setting the shoulder back too far in reloading will cause excess headspace. That is not true. The only advantage of keeping resizing to a minimum is greater case life, since there will be less flexing of the brass at the case shoulder. But that has nothing to do with headspace.

*A recurring problem with the .35 Remington!

Jim

 

[44 AMP]

With a rimmed case, if headspace is correct, case separation is almost impossible

Its not as "almost impossible" as you would think.

If the chamber is too large or the case sized to be too small, the case will simply expand to fill the chamber.

This is absolutely true, but this is also when the case can separate. I have had complete case head separation in SMLE .303 rifles. It absolutely can happen.

The headspace can be correct, (proper dimension for the rim), and the chamber/case size relationship can still be excessive enough for cases to fail.

That kind of thing is seldom a problem when firing a round ONCE, as usually cases will not fail the first time. Usually. Reloaders, on the other hand have to pay close attention to avoid issues.

 

[Jim Watson]

Neither of MY .35 Remingtons had any trouble holding headspace, nor a friend's that I shot a bit.

The normal recommendation for belted cartridges is to treat them like rimless and resize them to "headspace" against the shoulder.
Routine sizing back to factory dimensions is said to lead to case separation in front of the belt, just like a rimless with the shoulder set back too far, too often. I can't say for sure, I don't belt.

 

[James K]

If it is necessary for rimmed and belted cases to be "headspaced" on the shoulder, how come the headspace gauges are only short plugs and not full length.

As to case separation, I too have had separation in a .303, but it was due to excess headspace, and I suspect yours was also. Note the disclaimer in what I wrote. I said "if the headspace is correct", the case will blow out to fill the chamber instead of case separation taking place. The pressure expands the thin front of the case outward and forward without stretching the case enough for separation. Separation is caused when pressure sticks the front of the case to the chamber walls, but the bolt can move back enough to allow the rear of the case to pull back and tear the case apart. I suppose it could happen some other way; I know of none, but am willing to be educated if someone comes up with a reasonable explanation (not just "it happens").

As to .35 Remington, one of the big PITAs as a gunsmith, was the parade of guns in that caliber (mostly Marlins) that had erratic firing. It took a while to figure out that a chamber that was even a bit "loose" (cut with a new reamer, I guess) would allow a slightly undersize cartridge to "cushion" and cause misfires. The occurrences were random. Sometimes, we could correct the problem by fiddling with the firing pin, other rifles had to be sent back to the factory.

Jim