undersized die = reduced case capacity?

[ErikW]

Does an undersize sizing die cause reduced case capacity (and thus higher pressures)? Has anybody noticed higher velocities upon switching to undersizing?

Obviously it would reduce the cases capacity to hold a volume of water, however little. But what about when the cartridge is fired and the brass expands to the chamber?

The reason I ask is I'm trying to develop a load with MG 180 FMJ bullets and N330. A few years ago, I chronographed 5.7 gr at a wimpy 850 fps. After checking that note, I loaded up 6.0 and 6.2 gr. The 6.0 gave me 1032 on the first shot and averaged a hair under 1000, with 6.2 averaging a little over 1000. This is through a slow barrel that has had several thousand rounds through it since chronographing the 5.7 load. The only thing that has changed is the lot of N330 and the undersized EGW die.

[Nolan]

The only thing that has changed is the lot of N330 and the undersized EGW die.

And the temperature, barometeric pressure, phase of the moon, angle of the sun, and any other number of things.

The only way that I see an undersize die affecting velocity is with bullet pull. The smaller case mouth resulting from the undersize die holds the bullet longer for higher pressure. Even then I don't see it making a major change in velocity, 3 tenths of a grain difference causing an increase of 150 fps across different lots of powder is more likely.

[ErikW]

Well, yeah, of course the mysteries of Chrono Voodoo goes without saying...

Good answer on the bullet pull.

[D.Hayden]

On my .45, I just went to the 'U' die, there was no noticeble difference.. average of 10 shots strings were within 20fps.

maybe the .40 would be more noticeable withit's higher preasures?

[DogmaDog]

Interesting question.

I started right off with a U die for my .40s, so I have no data to test your hypothesis. Sounds reasonable, though. If it does happen, then you'd expect the effect to be more pronounced with smaller cases (a .001" reduction in diameter is a larger proportion of the total for smaller cases). The case tension on the bullet factor is reasonable, too.

DD

[PistolJim]

For me in .40 I noticed no real difference when I switched from Dillons size die to Egws. In .38 Supercomp I did notice a difference. It seemed the same load averaged about 4pf to 6pf faster with the u-die. Thats my experiece at least.

Jim

[Larry White]

Eric, I think that as the case is belled and expanded during the powder drop process the issue of case pull is taken out of play. I have used the U die for a year and am still useing the same loads without suprises. I have not used 330 so dont have enough experience with it to know what to expect from it, but I think your change of powder lots and conditions have more to do with your findings than the small reduction in case volume. Larry

[blkbrd]

lkytx hit on a good point, to get a much tighter bullet pull you will need to polish down your expander die.

Being the chrono fiend I am I have noticed a slight increase in velocity going with an undersized die and polished expander, small enough that it could have just been the phase of the moon? 4-6FPS is phase of the moon stuff.

However I have seen a dramatic drop in standard deviation when increasing the case tension on the bullet. This could be due to the standardization of the cases due to cold working, as a primer is enough to expand the case and push a bullet into the rifling. So I don't believe it is a reduction in case capacity, your chamber will affect the case size during fireing not the case size, just think about the fireforming process using rifle cases. A tight chamber will increase velocity.

JM2C

[Skywalker]

Erik,

I do believe that (as blkbrd pointed out) the biggest change switching to a U-die from a normal one is on the SD (direct consequence of more neck tension on the bullet).

My only explaination on why .3 grains more powder gave you such a big difference in velocities is that the actual N330 powder is different (maybe) from the one you tested before. I mean, they could have changed the powder composition over the years, or the compared lots were very inconsistent among themselves.

I only had a minor experience with that powder, but when I tested it in .45" (200 grs SWC and RN bullets) some 4/5 years ago it gave me results almost equal to N340 (i.e. same powder weight -> almost same velocity, +- 20 fps difference between powders).

[.40AET]

Eric-Loading for .40, I used 5.7g of N330 with MG 180g RNFP and was getting 170pf, with small rifle primers. If I switched to small pistol, it only squeeked by or not at all. This was all with a EGW "U" die. Sorry, I have no data for a normal die.

[ErikW]

.40AET, was that at 1.20"? What brand primers? I'll have to check my notes on whether I used WSP or WSR at the time, but they generally don't change the velocity of my rounds.

[.40AET]

Sorry Eric- 1.20" oal with CCI primers. I was using WSP and they too had lower velotity that small rifle primers. I called CCI last year when I was having some ignition problems with my powder. I was getting some unburned powder leaving the barrel and usually spraying on my arms. The small rifle and small pistol magnum primers have a longer flame which is used to completely burn all of the powder in larger cases. Going to small rifle primers fixed the ignition problems right away. When I chronoed the rounds, the same powder charge was also giving me about 40-45fps more velocity. I backed off the powder charge, made 170pf, and never looked back. Try 10 rounds with WSP and 10 with WSR. I bet the 5.7g and WSR will give you 170pf.

[ErikW]

I just chronographed loads from 5.7-5.9 gr and re-chronographed the 6.0 and 6.2 gr loads for a sanity check. The slowest round from 5.7 was 926 fps. The 6+ gr loads were as before.

Maybe Vihtavuori isn't so consistent lot to lot after all.

P.S. 183 PF with 180s is very unpleasant.

[ErikW]

This is really weird. The 180 FMJ N330 loads making 170 PF are getting blow-by, where the case doesn't fully seal and one side gets sooty. (And rounds in the magazine get discolored by the gas blow-by, so it must be significant.) The (rifle) primers don't look anything like the primers fired in N320 loads. And there's a little more muzzle flip and a lot more sting felt in the hand. All this from a powder that's only slightly slower.

[Jim Norman]

VV N320 5.1 Grains, 180 Zero JHP, 1.18 OAL 171 PF, WSR Primers, No preasure signs. I may cut back a bit to 5.0 and get down to 169.00 PF

Para with 40k rounds plus, EGW Barrel

YMMV