Shades of brass

055

We’re almost on to neck sizing, but there are a couple of full length (FL) sizing issues worth mentioning.

First, I’ll return to that perennial question: Which is ‘better’, FL or neck-only (NS) sizing?

While benchrest competitors traditionally neck-sized their brass at the back of the firing point between relays, an increasing number now FL size it on portable presses with a custom die.

Harrell’s Precision makes an excellent range of small C-form presses alongside bushing sizers for the 6PPC and 6mm BR.

The die-body cavity is reamed to a range of marginally different dimensions – send a fired case from your chamber and Harrell’s supplies the version that is the closest match.

Why do these obsessive seekers of ultimate precision use this method? They often get their five shots off really quickly, in 20-30 seconds, if wind conditions are stable.

Easy, smooth cartridge chambering and rifle operation are critical. The tiny amount of body sizing that takes place in the custom die helps achieve this and at the very least doesn’t increase group sizes, maybe even reducing them.

 

A portable Harrell’s Precision press and custom FL bushing sizer for 6PPC

A portable Harrell’s Precision press and custom FL bushing sizer for 6PPC

Form and Trim

Another form of FL sizing is case forming, where a case for one chambering is radically resized to be used in another.

In extreme examples, more than one die is used, doing the job in steps, but a single die often suffices.

The photographs show one such die: an RCBS form and trim die that turns 7.92x57mm Mauser or .30-06 Springfield cases into 7.65x53mm Mauser, pushing the donor case shoulders back and imparting a new body taper.

On sizing, the now excessively long neck on the reformed case protrudes from the die, and is cut down flush using a hacksaw and file.

The case mouth is cleaned up and chamfered, the neck expanded, and it’s ready to load and use.

Necks

I hope I’ve shown that FL sizing is often not as straightforward as it seems, and neck sizing is no different.

How you size necks, and by how much, is important. The fit of the bullet is absolutely crucial to ammunition performance, as is its concentricity, which I’ll look at next month.

The RCBS form and trim die with a reformed .30-06 case before the excess neck is cut off (the die top is hardened and is unaffected by a saw blade or file)

The RCBS form and trim die with a reformed .30-06 case before the excess neck is cut off (the die top is hardened and is unaffected by a saw blade or file)

By ‘fit’ I mean neck tension, the degree of grip the neck walls exert on the bullet shank, and that relates to actual amounts, as well as consistency between rounds.

I prefer light tension, so little that hardly any effort is needed to seat bullets on my Forster Co-Ax press.

I’d say most standard dies over-do the degree of neck sizing, over working the brass and leaving an over-tight bullet fit.

Fortunately, we can decide how much neck tension we impart thanks to variable dimension dies.

Obtained through interchangeable neck-bushings, their inside diameters are usually available in 0.001-inch (1-thou’) steps.

Such dies, whether FL or NS types, can seem expensive, but are increasingly chosen by precision-minded rifle shooters, not just those who only shoot at paper targets.

Working the Brass

Despite their high initial outlay, bushing dies may provide savings in the long run – especially for those who shoot a lot, frequently resizing and reloading their cases.

Bushings reduce the amount the brass is worked, thereby extending its life.

The amount you ‘work’ necks in the firing/reloading cycle is partly determined by chamber dimensions: How much clearance there is around the neck determines the degree of expansion on firing, and by how much the die sizes the neck down before re-expanding it again to the final setting.

Reforming .30-06 (left, before and after sizing without trimming); 7.92x57mm (centre and second from right) into 7.65X53mm Belgian / Argentine Mauser (far right)

Reforming .30-06 (left, before and after sizing without trimming); 7.92x57mm (centre and second from right) into 7.65X53mm Belgian / Argentine Mauser (far right)

You cannot do much about the former short of replacing the barrel with one incorporating a tighter chamber, but you can transform the second part.

Let’s look at two .308 Win chambered rifles I handload for, a long range Savage PTA-based target rifle with a minimum dimension chamber and a Howa 1500 Varmint sporter with its chamber the same as it left the factory with.

Their cases have a neck wall thickness close to 0.015in, so loaded cartridges have neck O/Ds of 0.308in (bullet) + 0.015in + 0.015in (case) = 0.338in.

Fired examples measure 0.339-0.340in for the tight-chambered Savage and 0.345-0.346in for the Howa, giving the latter’s necks a much greater amount of work during firing and returning them to their original dimensions – 2×0.008in against 2×0.002in for the Savage.

The true values are higher as there is a thou’ to thou’ and a half ‘springback’ from the fully expanded position as chamber pressures return to zero.)

So the Savage cases see about 0.007in total change against 0.018-0.019in for the Howa’s. That’s before we get into sizing the neck down to less than bullet diameter to provide the essential neck-tension/grip on the projectile.

I use a bushing die on both rifles’ ammunition, but let’s pretend I only use it on the target rifle, and the Howa’s brass gets a run of the mill Lee FL sizing die.

Redding ‘Type S’ FL bushing sizer die for .308 Win (left) with a ‘Competition’ seater die

Redding ‘Type S’ FL bushing sizer die for .308 Win (left) with a ‘Competition’ seater die

I select a 0.336in bushing for the former adding another 0.002in of brass movement to the process (for a total of 0.004in once the bullet is seated, which expands the back neck to 0.338in).

Taking the Howa, we now remove the Lee die’s expander and FL size some cases for it, measuring their neck outside diameters before expansion.

Doing this reveals that the die sizes the neck down to 0.3315in, but we mustn’t forget ‘springback’. So the case neck is reduced from somewhere around 0.347in in the rifle chamber to 0.330in by the sizing process, losing 0.017in.

It then has to be re-expanded by pulling an expander ball or ‘button’ through it to bring it back up to 0.335in O/D. Finally, add another 0.003in for the expansion caused by pushing a bullet into the neck to return it to its starting point of 0.338in.

Brass used in the tight-chambered custom built rifle and sized with a bushing die undergoes a total ‘movement’ of around 0.011in, ending up with very light neck tension.

The Howa would see ~0.034in total change, three times as much.

Sizing the Howa’s cases in the bushing die would reduce that to 0.022in, which is still a lot but a worthwhile reduction of more than a third, and with better control on the degree of tension on the bullet.

Laurie Holland

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Posted in Ammunition, Features, Reviews

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