I have a 527 American in 223 that I'm thinking about re-barreling. I haven't decided yet whether to go 6mm or .257. I'm interested in getting an 85-90 grain bullet to about 2700-2800fps. This is just a pet project I've been thinking about for a while and though I've always planned on using the 223 case necked up, I'm now considering the 204 case as well.

I'd like to simply neck up without fireforming. It looks like I'd get a little extra case capacity with the 204 but I'm wondering if it has a solid future. So I fall back on the good ol' 223. Then I think about the extra case capacity of the 204 again...

Any recommendations or other points to consider from you folks would sure be appreciated. I'm ultimately interested in a light walking-around rifle for pest control out to 250-300yds and maybe the occasional small Blacktail out to 150yds if the opportunity presents itself.

The 204 case offers the most case capacity of all the 223 sized cases. There's not much difference in price in buying commercial 223 or 204 cases bulk. Good luck in your project and keep us posted on how it works out.

if you look in the hodgdons powder manual #27 there is a listing for the 257 kimber. it is the 222 rem mag improved and necked to .25 since the 204 is essentially the same except for neck diameter that would be a good place to look.
i've often thought that this round pushing a 90 gr sierra @ 3000 fps would be very practical and effective.

Thanks for the responses guys. I was just looking at some 204 brass today at Sportsman's Warehouse and thinking that it ought to neck up to 25 caliber pretty painlessly. I'll check up on the 257 Kimber ballistics as well. That 90 gr. Sierra is one of the bullets I've been thinking about.

Then I think I'll have to check into what it will take to get a new 22-inch factory contour barrel put on. Maybe I'll give Pac-Nor a call next week.

It's too bad that CZ uses a different receiver and magazine for the 223 and 7.62x39 versions of the 527. I've been working on a few cartridges for the 527 using the Czech 7.62x45 as the base catridge. Looks like my 6x43 CZ is exactly what you want, but wrong case head size.

http://guntech.com/ammo/6x43cz.jpg


Cartridge : 6x43 CZ
Bullet : .243, 85, Sierra SP 1520
Cartridge O.A.L. L6: 2.260 inch or 57.40 mm
Barrel Length : 20.0 inch or 508.0 mm
Powder : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.124% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms

-11.2 86 28.23 2616 1291 36675 9343 94.2 1.164
-10.1 87 28.58 2648 1323 37921 9477 94.8 1.147
-09.0 88 28.94 2680 1355 39209 9608 95.3 1.129
-07.9 89 29.30 2712 1388 40538 9734 95.8 1.111
-06.7 90 29.66 2744 1421 41917 9857 96.3 1.094
-05.6 91 30.01 2776 1454 43340 9976 96.7 1.077
-04.5 92 30.37 2808 1488 44812 10091 97.1 1.060
-03.4 93 30.73 2840 1522 46335 10201 97.5 1.043
-02.2 94 31.09 2872 1556 47910 10306 97.9 1.027
-01.1 95 31.44 2903 1591 49540 10407 98.2 1.011
+00.0 96 31.80 2935 1626 51226 10502 98.5 0.996 ! Near Maximum !
+01.1 98 32.16 2967 1661 52971 10593 98.8 0.981 ! Near Maximum !
+02.2 99 32.51 2998 1697 54778 10679 99.1 0.966 ! Near Maximum !
+03.4 100 32.87 3030 1733 56650 10759 99.3 0.951 ! Near Maximum !
+04.5 101 33.23 3061 1769 58591 10834 99.5 0.937 !DANGEROUS LOAD-DO NOT USE!
+05.6 102 33.59 3093 1805 60597 10903 99.6 0.923 !DANGEROUS LOAD-DO NOT USE!

You're right, those ballistics look ideal.

I built a 6x47AI, 22 mag case necked to 6mm and blown out to a 40 degree shoulder 1-12 twist and shoots the 87 gr hornady in your expectations. Very quiet easy to shoot and have only shot whitetails and varmits with it. A short light rifle that fit the bill to keep in the pickup at the farm.