Small Base Dies

[GrocMax]

When do you NEED a small base die? When the chamber has been reamed with a small base reamer, PTG includes small base feature into a lot of 223 based reamers, 20P being one of the most common.

Most new brass comes in at .373" base diameter, SB chamber is ~.375", SB die gets brass fired in a std .377" chamber back to ~.374". Some of the military chambers can be quite large diameter approaching .380" case diameter on extraction, not worth reusing that stuff.

While somewhat of a PITA if caught unawares, the small base appears to keep my primer pockets tighter longer, only makes sense as less case head expansion happens.

 

[dolek]

Run a reamer in there, and be happier.

Nah,that would be crazy...my chamber and dies are a "Matched" set.

 

[Martin52]

Anyone know what the case head dimension would be for a 0.308 Lapua case where a small base die is used ?

I have a few cases that I need to reduce, I think, to around 0.467, my standard FLS die gets me to 0.469 and they're sticking in the chamber.

thanks
Martin

 

[Texas10]

If you have unequal case wall thickness the case will bulge on the thin side and warp. And this can cause accuracy problems and why full length resizing eliminates the base of the case body from effecting bullet alignment with the bore.

I say this because I have seen Remington cases with .009 thickness variations that warped badly when fired. Meaning the quality of your cases will greatly effect their concentricity after firing the first time.

"a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway.

In conclusion, I believe that allowing the bullet to find a relatively stress-free alignment in the throat by full length sizing (including the neck) and turning necks to enhance concentricity gives the bullet the best probability of a well-aligned start into the rifling."

The Rifleman's Journal
Germán A. Salazar

If I'm not mistaken, that is what's referred to as "the rat turd in the violin case" theory?

I do check for "banana shaped" case deformation, but not really having that issue and accuracy is apparently way better than my shooting skills. First group in the zeros or low ones, next is high twos to mid threes. The nut behind the bolt seems to be the biggest issue.

 

[fatelvis]

In my 35 yrs of handloading, for many different rifles with different actions, I've never saw the need for a SB die. Then again, that's in my limited experience.

 

[dolek]

Never needed one either,but use them for some rifles with tight chambers and less than stellar extraction . No set rules on the subject . I do know for sure a stuck case will ruin your day ,so why take a chance if there is any question ? I also use them for sizing brass from my ARs that will be used in other rifles .

 

[Bc'z]

Never needed one either,but use them for some rifles with tight chambers and less than stellar extraction . No set rules on the subject . I do know for sure a stuck case will ruin your day ,so why take a chance if there is any question ? I also use them for sizing brass from my ARs that will be used in other rifles .

Exactly, cases fired in wife's AR will not chamber in mine without use of a SB die.

 

[Dusty Stevens]

Br shooters use them once match brass gets hardened

 

[BoydAllen]

If you start with new brass and keep it dedicated to one rifle, you will not have the "previously fired in a larger chamber" issues.

Another thing that can cause issues, even when starting with new brass is when the back of the chamber is too close to the size of new brass. Even though the rounds chamber smoothly, the bolt will get tight a a lower charge than if there had been more clearance on a new case.

A friend experienced this when he switched his BR from Remington brass to Lapua. His smith polished out his original chamber to the point where new Lapua (that was shortened due to the difference in neck length) would chamber just fine, but he ran into tight bolt issues sooner than he should have, going up gradually with his charge weights. After that, his smith recut the chamber with a reamer that was correct for the Lapua brass, that had more clearance on a new case, and it solved his problem. There has been a general thought that has been around for years that a closer fit is always better. IMO that is not the case. As long as the FL die is correct for the chamber, you are better off with a back of chamber dimension that has a decent amount of clearance with a new case.

Years back, at the request of a friend who I knew to be a meticulous reloader, I helped him by designing a .300 Weatherby chamber reamer that had a tight neck, and the freebore dimensioned so that the bullets that he preferred for hunting could be seated into the rifling and still fit in the magazine. Having learned from the other fellow's experience with a 6BR, I left the body diameter specifications the same a stock Weatherby chamber. Obviously I needed to do a pressure investigation, since there was no reloading data for the chamber I had designed. This was done loading at the range, using a single case. We ran the pressure up well above what I would recommend, based on more than one pressure sign. At the end of the test after letting the case cool for a bit, it was not tight when chambered, even though all of the sizing had been done with a neck die. I attribute this to the generous chamber body dimensions.