Gunsmithing Reamer specs to brass clearance question.

[wadcutter]

I hope you lads can give me some advice on reamer specs.

I just measured up some new Bertram .375 Chey-tac brass and compared it to the Reamer print I have.
Reamer base diameter 0.6419”, Bertram Cases measure 0.634” so about 0.008” slop in the base end of the chamber.
Reamer diameter 0.605” at shoulder. Bertram Cases measure 0.595” so 0.010” slop in the shoulder end of the chamber on new unfired brass.

Is this sort of clearance excessive for best accuracy?

Now I am going to order a new reamer for a wildcat cartridge using this brass. I would like to keep the clearance to a minimum for match grade accuracy out of my Stiller 408 single shot action. So how much clearance should I specify to provide correct function while keeping the chamber as tight as practical.

Thanks for your advice.

 

[excaliber]

If you want a "match chamber", get reamer made from the specs of the brass. You will find that very precise gun builders do this. Then have gunsmith
make seater die with the same reamer that made your your chamber from scrap barrel using Wilson inline die style and use K&M arbor press with
gauge to measure neck tension. Then have a full length sizer die with correct bushing made.

 

[Edds]

So how much clearance should I specify to provide correct function while keeping the chamber as tight as practical.

PTG shows their standard resize reamer to have a .0035" smaller diameter than a finish reamer on the body. You specify the neck size.

 

[RonA]

I went ahead and bought dies, but i plan on running a few rounds through mine and then making making a neck sizer and f/l to match the chamber.
Kind of backwards, but it's all I can do now.

 

[Straight Shooter]

The brass is built to really loose tolerances and it varies batch to batch. If you were to min spec a reamer based on one lot of brass there is half a chance you may never find brass that will fit again. It really doesn't make that much difference on the case body how loose it is. Watch the neck. There is a little to be had there.

 

[turbo54]

The brass is built to really loose tolerances and it varies batch to batch. If you were to min spec a reamer based on one lot of brass there is half a chance you may never find brass that will fit again. It really doesn't make that much difference on the case body how loose it is. Watch the neck. There is a little to be had there.

^^this seems like good info.

Seems like leaving a little breathing room in the chamber to accommodate varying lots of brass would be a good idea.

I'm no benchrester, but I know I've seen virgin brass shoot frickin GREAT out of a loosey goosey SAAMI chamber and that leads me to believe" tight" chambers offer little accuracy advantage.

My 284 has a tight "match" chamber, and it's a pain in my ass. Actually, neither a Forster or Redding FLS die will size my brass quite enough down at the .200" line, leading to sticky primary extraction. Luckily, Redding makes SB FLS die which solves the problem, though it sizes the body more than is needed.

My lesson learned: A GOOD, PROPERLY DIMENSIONED sizing die is at least, if not more important than the chamber dimensions. PAY ATTENTION to the compatibility of chamber reamer and die reamer!

I would fully intend on ordering a die reamer at the same time as the chamber reamer.

 

[wadcutter]

That's the idea but I was looking at how tight I could go without getting into any problems like those mentioned above.
I could buy enough brass to last the barrel and make the reamer to suit that?

 

[SPAK]

I've toyed around with making a reamer .002" larger in all body dimensions other than the neck based off of new lapua brass for one of my rifles. The thought process was to minimize brass expansion and have a chamber matching new brass pretty close to optimize accuracy from the first firing forward.

However, something else to consider is your internal case capcity will likely be less giving less room for powder. I'm not sure how much less but I do know that a difference of .001" on the body and .002" in neck diameter made a difference of .8gr of powder for my .30-06 using lapua brass.

Your specs are showing quite a considerable difference between new brass and chamber size. I would imagine that the impact on internal case capacity would be greater. On the same note in your particular caliber there is a lot more case capacity to begin with so percentage wise the impact might not be that significant.

Once you fire your brass, it will form to your chamber pretty well and you won't really have to worry about the slop. You might even find that your new brass will shoot just as accurately as your fireformed brass anyway. I wouldn't worry too much about the body specs, I've seen about the same if not more difference in size with some of the .338 Norma mag reamers I've looked at and those seem to be shooting quite well.

On some rifles I do specify my neck diameters a little tighter than some with about .004" of neck clearance on a loaded round to minimize neck expansion. Some would say this is too tight and could cause problems in the field. I do agree that there is some risk involved in making any of your chamber specs too close a match to your brass if you're taking your rifle into the field. A little bit of dirt in the chamber or on your rounds could cause problems if you don't have enough clearance.

 

[SPAK]

^^this seems like good info.

Seems like leaving a little breathing room in the chamber to accommodate varying lots of brass would be a good idea.

I'm no benchrester, but I know I've seen virgin brass shoot frickin GREAT out of a loosey goosey SAAMI chamber and that leads me to believe" tight" chambers offer little accuracy advantage.

My 284 has a tight "match" chamber, and it's a pain in my ass. Actually, neither a Forster or Redding FLS die will size my brass quite enough down at the .200" line, leading to sticky primary extraction. Luckily, Redding makes SB FLS die which solves the problem, though it sizes the body more than is needed.

My lesson learned: A GOOD, PROPERLY DIMENSIONED sizing die is at least, if not more important than the chamber dimensions. PAY ATTENTION to the compatibility of chamber reamer and die reamer!

I would fully intend on ordering a die reamer at the same time as the chamber reamer.

Turbo brings up some good points about the sizing die being a good fit for the chamber too. The other option is to call up Forster or Redding and see if they will send you a print of their die dimensions. They may or may not, but they might be able to tell you what finishing reamer print they base their die off of. This way when you go to order your reamer you can have an idea of what dimensions your sizing die will size to.

 

[wadcutter]

Thanks Frogman, some good thoughts!

 

[SPAK]

Thanks Frogman, some good thoughts!

No problem, keep us posted.