Rimmed cartridges and 'headspace'

[AJMD429]

I found a gunsmith who will take off my reluctant Marlin 1894 barrel for $75 or so, and decided to take him up on the offer, just to save myself the time and the risk of destroying a perfectly good .44 Mag barrel.

He says (and I TOTALLY understand it, since I'm a physician) he will not put on the new barrel without checking headspace, and he didn't have the gauges, so I just ordered a set (since I want to keep an eye on my .454 Casull Rossi).

My question is - with a 'rimmed' cartridge - what would actually happen if 'headspace' were too large, or too small. . . ? ? ?

I don't understand how the cartridge slipping back a few thousandths of an inch would really cause a problem. Edjucate me...

[Buck Elliott]

Too much headspace in a .44, or better yet -- a .454 -- allows the case head to move back against the bolt, after being driven forward by the firing pin..the case walls are momentarily "stuck" to the chamber, by pressure/friction, while the case head moves back until it meets some support.. In a levergun, that would be the bolt; in a revolver, it is the standing breech.. in that process, something has to give, which is the case wall, closest to the head, stretching and thinning that portion of the case.. Repitition of the process leads eventually to case-head seperations, and ugly gas blowback, which can be anything from annoying to dangerous..

The farther the case can 'run' before it contacts the bolt, the harder the impact on the gun's lock-up..the harder, and more often you slam those parts, the more likely you will see accelerated wear, and loosening of the gun.. In a cartridge such as the .454, which CAN run pressures as high as 65,000psi, you begin to visualise some of the problems waiting in the bushes, down the trail...

Make sure you have no more than .002" - off the gauges - in your leverguns.. Actual cartridge dimensions will give you just a tad more room...

Too LITTLE "headspace" will not allow the bolt to close completely, or lock up properly...

[AJMD429]

Thanks for the answer - makes sense.

I went ahead and got the gauges for .45 Colt/Casull, but have heard people with leverguns using 'Scotch-tape' method to see if things were too far off - are they just doing that with loaded factory rounds, or brass fired in 'known-to-be-ok' guns, or what...?

Also - it seems like if headspace weren't 'too' far off (whatever that would be), and you ONLY used the brass reloaded for that particular gun, things would not get all that bad, since you'd have only the one-time stretching, if you only neck-sized.

Sorry for the novice questions, but I don't want to blow up a gun I spent good money on, and there's that fingers and eyeballs thing, too. . .

[Lastmohecken]

If it's a bottle neck case like a 30/30 you could adjust the resizing die to headspace on the shoulder instead of the rim but on a 44 mag, the rim is pretty much what you headspace on, and If it the barrel is not headspaced right, using the same cases won't make much difference and you are going to keep stretching the case and thinning it out, but I would think it's the rare gun that would would have a problem.

On some guns, and calibers, you can change the headspacing to the bullet contacting the lands in the barrel. The old shooting manuals used to talk about this on the 45acp and I did it for years with thousands of rounds. I would use a heavy roll crimp on cast lead bullets and seat the bullet out to contact the lands in the barrel and never had a problem but later went to a taper crimper and got away from it, but it worked fine for many years for me and was accurate too. I shot my brass many many times and only threw a piece of brass away if the mouth became cracked or a side split out of a case, granted we are just talking about the 45acp, not a high powered rifle.

[Griff]

Excellent explanation Buck. Even I followed along.

Now, for the bad news; both the chamber and your brass have manufacturing tolerances. Lets continue with the .44 Mag Doc is interested in. SAAMI sez that for the chamber to be "in spec", the relief for the cartridge rim must be between 0.060"-0.070". Likewise SAAMI sez the cartridge rim should be 0.060"-0.011

So, unless I'm mis-reading something, you could have a max chamber of 0.070" and if your brass rims were runnin' right @ 0.060", you'd be exceeding the .003-.007" headspace most recommend, yet be "in-spec". 'Tis why I prefer the Plasti-gage method of checking headspace over "go-no-go" gages for rifles. For revolvers, I don't know any other method. I can measure the rim thickness of my brass, add the squashed plasti-gage dimension and know immediately what the actual dimension my brass must be to keep my headspace within the recommended range.

Now, do I do that? Not ALL the time, but do whenever I get a new bunch of brass... With my .45 Colts, not so much, as I'm runnin' those at light moderate velocities, less than 1,000fps outta a 24" barrel. But, my .30-30s, yep, I'll sample 50-75 cases out of a thousand. If those run pretty consistent, I'm good. When I've picked up range brass, I tend to be pickier. I prefer brass that was from new ammo, verifiable as "once fired".

Since most of my .30-30 shooting is done with one specific rifle, and it tends to split necks after the 3rd or 4th reload, I don't have to worry about too much about other indicators.

[Nate Kiowa Jones]

I once heard headspace explain like this. Imagine a bowling ball loose in the back of your pickup truck bed. That truck moving starting and stopping will cause the bowling ball to batter the inside of your bed for sure. But, if you have that ball in a sturdy well fitting bowling ball bag it will not be able to get a running start to do all that damage.

Ideally "0" headspace is the best but for leverguns that is not practical. As for using go- no-go gages I hardly ever use them for rimmed cartridges. They don't really tell you what the actual headspace is, just whether it's acceptable or not.

Tape, clay or even plasi-gage can be iffy, too easily distorted. Particularly if you don't remove the ejector (spring loaded, bolt mounted like the wins) and extractor.

Still a shade tree method but a better way is to use depimed brass and start by trimming it in a case trimmer so that it is even. Then measure the overall length of the brass. Next re-seat a spent primer about half way seated then chamber and close the bolt using the lever to press the primer on in. With excessive headspace the primer won't fully seat. Re-measure the case and subtract the first measurement. Because of variances in rim thicknesses it's still not the best but it will definitely show serious excessive headspace.

[Cliff]

Just wondering if barrel harmonics contribute much and how any head space problems might affect those harmonics. Bolt guns do not normally have to deal with stuff like magazine tubes, forearm attachment and so on. Seems some years back a number of shooters were all concerned about harmonics and barrel "nodes" and were trying to tune a barrel. A number of people get all excited about headspace and it is important if you reload or expect proper functioning of the firearm in question. Normally a big head space can work (not well) if you are not concerned with reloading or good accuracy. Just saying.