Using boolits <groove diameter

[Glwenzl]

I am loading .317 boolits for an old 7.62x54R that has a huge .316 groove and recently came across the following info which if I’m reading out correctly, has me concerned about using these boolits and confusing about chamber limitations requiring .310? Thought I’d run it past you guys to see if there is something here I need to better understand about chamber limitations.

[omgb]

Well, my MN easily chambers .313 with 180 grn bullet and 25.2 gems of 5744 I get no pressure signs.


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[hoodat]

I think they may be concerned about cramming a larger diameter bullet into a neck and throat of a chamber which isn't cut for the larger diameter.

A chamber cast is never a bad idea if there is any question, and

is often revealing. It seems unlikely that the groove diameter would be larger than the throat or the neck.

You DO NOT want the neck of the chamber to be smaller than the dimension of your loaded cartridge.

One of the great advantages of reloading, is that we can do a few things to "custom fit' our ammo to work better in the chamber and barrel of a particular firearm. jd

[Larry Gibson]

Some ComBloc MNs have groove diameters almost as large as or close to the chamber neck diameter. Those chambers most often do not have a "throat" with a step at the front of the chamber neck. Thus they ended up with wartime expedient groove dimeters quite large. Many such chambers will not allow the necks of cartridges loaded with groove diameter bullets because adding the thickness of the neck dimeter makes the cartridge too large to chamber.

A simple chamber cast of your rifles chamber will quickly tell you what the diameter of the chamber neck is. That way, knowing the thickness of the case neck (times two) subtracted from the chamber neck diameter will tell you the largest diameter of bullet you can use. It also will give you the groove dimeter at the leade, the length of the throat and whether or not there is a chamber case mouth. Parafin wax works well for this. If you don't know how to make a chamber case ask?

[Glwenzl]

Thanks, some of it is beginning to make sense.

I have done a few chamber cast and still have enough Cerrosafe to make one more (maybe not the entire chamber but the important part I need right now). Makes me wonder if there is any of the leade/throat there. Beginning to sound like the lands and grooves may be pretty abrupt. The bullet is pretty blunt, a PC Lee CTL312-160-2R

[truckjohn]

Many MN's have very tight chamber necks. So while the bore might swallow the bullet, you can't stuff the loaded ammo into the chamber. 0.314" is about the largest I can reliably chamber, and that's with neck turned brass.

Guess how I found out.

Anyway, you can easily measure fired brass necks to get an idea, or just slug the neck with a fat fishing sinker.

[Glwenzl]

Thank you. This is my son-in-law’s rifle that I am playing with. He had no brass it was all steel cases and I’m not very familiar with them. but as it turns out the fired cases are all measuring .337”-.338”. They have .010”-.011” neck thickness and so does the new brass I purchased for it. My loaded boolits measurements are pretty much the exact same size s as my fitted steel cases (.337-.338).

Tomorrow I will fire some of the .312” jacketed bullets I loaded up and see if the brass measures the same as the steel cases I have. If so I’ll pull boolits and have to size smaller.

[samari46]

Finnish Moisins usually have smaller rifling dimensions that the Russian ones. I shoot lyman's 314299 sized at .3135 out of my Finn model 27 which slugs out at .3115. I bought that mold shortly after it came out and it has a .303-.304 bore riding nose and .314 body. My model 27 liked that bullet so much I never tried anything else. And bought the same mold to keep as a spare. Cast with it a few times and dimensions are the same as the original mold. Frank

[truckjohn]

Brass cases are often thicker than steel cases. I expect 0.012-0014" neck thickness with brass vs 0.010-0.011" on steel. So if your fired case neck mic's 0.338 and you have 0.013" thick brass and 0.002" for chambering reliably, that leaves room for 0.310" bullets. (0.338-0.013-0.013-0.002 = 0.310"). The same math on 0.010" steel cases gives you room for 0.316" slugs.

[pworley1]

.314 has worked fine in the 91's and 310 for the 39's.

[Bigslug]

If your inside neck diameter of a fired case is .337", then the outside diameter is probably pretty close to .357", and that's after the case has de-pressurized and sprung back. Sounds to me like you have room to spare for the .317"-.318" bullet your .316" barrel would call for.

If I read you wrong and .337" is the outside diameter after springback, that still makes for an inside diameter of about .317" before sizing.

Either way, I'm not hearing any alarm bells or air-raid sirens.