Insufficient chamber leade

[dilly]

I've been having all sorts of trouble getting a myriad of bullets to chamber reliably in my Mini-14 300 Blackout. This has gotten me interested in looking at the dimensions of the chamber. It's hard to see from this picture, but it seems to me there is practically no leade into the rifling. I wish I could get a better picture but picture is as good as I think I can do.


Does this look normal to you guys?


162249

By the way, I know there is some confusion about the term but by leade I mean the part of the rifling that gradually ramps up from "freebore" to the full rifling depth.

[sdcitizen]

Quite difficult to tell from that photo, a pound cast is what you need. I had a 308 that was short and under diameter in the throat, ended up getting a throating reamer and am much happier now.

[leftiye]

Chamber cast - by whatever method. Turn a piece of rod to groove diameter seat it overlong and loosely in a case and chamber it as an alternative method of seeing what length freebore ends.

[Jim_P]

Roto Metals sells a chamber cast material that is low cost and you can melt with a propane torch. Only need to get to like 160 degree Fahrenheit. It does have a cooling and shrinkage curve but if you get it cooled down you have a few weeks to get accurate measurements.

http://www.rotometals.com/product-p/chambercastalloy.htm

[dilly]

Hm. Both good options. Tim Malcolm's pound cast seems like a better, more permanent way to get good measurements, and I have all the materials on hand. However, I am kind of worried about getting it stuck and having to sheepishly take it in to a gunsmith to remedy.

The cerrosafe stuff probably doesn't have much risk involved, as long as I can figure out a way to accurately pour that without overfilling.

[leadman]

there is a short window in which to obtain the measurements with Cerrosafe. IIRC you need to wait 1/2 hour after it is poured to take the measurements. Remove it from the chamber as soon as it hardens. I think it only a couple of hours to get the correct measurements. Should be instructions with the CS or look them up on Midwayusa website.

[NC_JEFF]

I pound cast, pure lead doesn't grow or shrink later in life.

[DougGuy]

Once you figure out the throat you might be able to rent a throating reamer from 4D rentals. If I had a .30 caliber reamer I would offer to do it.

[Ballistics in Scotland]

The picture does indeed look as if the throat is short, and a cast is the right way to check on this before laying out cash. But the slight shrinkage of Cerrosafe isn't of the slightest effect in determining this, and all of the usual materials used (lead pound cast, Cerrosafe, sulphur, candlewax, car body filler etc.) will be safe and satisfactory.

This cartridge is commonly used with 125gr. bullets of conventional design, often with quite bit of the parallel section exposed. If this rifle or its barrel have been made for you, without instructions to cut a throat unsuitable for this, your best recourse might be with the person who did it. If you do need to do it yourself, even if you have to buy the reamer, it is a lot cheaper than a chamber reamer, It must be a specialized piloted reamer, made for the purpose.

In theory you could make a tubular guide to fit the case neck and use an unpiloted engineer's reamer. But that tube would have to be very exactly dimensioned, and you get back to a chucking reamer having too steeply angled a chamfer at its front end, and a hand reamer not steeply enough.

[country gent]

One of the big changes when building a M1A/M14 or garand for national match use was shortening the throat to were mag length ammo started touching the throat. I have one here chambered in this manner and its a great shooter out to 1000yds. A tight throat dia is one issue and the length is another. The throat in a rifle is a short lived proposition as with use it gets longer How fast depends on use and cartridge intensity. If you burn 20 rd clips in 1-2 mins all day long throat wear will be greater than the same number off rounds loaded and fired single loaded. Do a chamber cast or pound cast and see what you have for chamber in relation to your cast bullets, If possible get not only the neck throat and leade but some of the bore as well, this will help determine if its throat dia, throat length angle, or a nose riding bullet with to big nose dia. If you are shooting jacketd bullets then its also how much these bullets infringe on powder capacity loaded to work.

[dilly]

The other question I have is, when I get the measurements, what types of modifications can be done specifically on the mini 14 type barrel? I would think this would be simple enough on something like an AR15 where the barrel comes off relatively easily, but I've never disassembled a mini 14 or M1A barrel from the receiver.

[sdcitizen]

Dilly, I ended up buying a 30 cal solid piloted reamer from Manson Precision, and made my own handle to turn it in the barrel of my m1a while still installed. It didn't take long and was easy to do, the biggest thing was re-casting the chamber to check progress till it was the profile I was after. Shots cast great now. I don't see any reason this wouldn't work on a mini 14 as well.

[dilly]

Last night my Rotometals chamber casting alloy came in. I plan to try and do the casting tonight.

The low melting temperature metal sounds kind of fun to mess with actually.

[akajun]

That looks like a normal 300blk chamber to me. THere is basically no shoulder, then the next line you see is the step for the case mouth, then most .300blk has a LOOONG lead so that you can fire 220gr bullets that are basically bore riders. That long lead is hard to see.
Rather than a chamber cast, I would get a Hornady stoney point guage and measure your throat.

[country gent]

Manson and several others make chambering reamers that can be driven from the muzzle end, check with them they may make a throating reamer like this also. They were used for setting final headspace on garands and M14s in armory rebuilds. It would save pulling the barrel and aquiring the barrel vise, blocks and action wrench to remove barrel.