.25x40mm AR-15 .257 Wild-Cat

[kurtz]

Had a couple people ask that I post some info on a .257 wild-cat that I've been working on...kinda like a shortened .25 copperhead for use in a AR-15 with 100gr bullets...

the .25x40mm (AKA: CottonMouth, named for all of the snakes that we pulled out of a friends Sabine Pass home after Hurricane Ike) is based on .223 parent brass shortened to 40mm and necked to .25cal to allow the use of 100gr Sierra MatchKings and GameKings set to mag length in a AR-15....the shoulder is slightly blown out and has a 30 degree shoulder....brass forming and reloading is pretty straight forward, should get range time with it next weekend, too many projects on the bench....

495496

Thanks,

kurtz

[JFettig]

Cool!

Can you post a pic of this cartridge next to a .223 and maybe a .300/221?

Jon

[kurtz]

Cool!

Can you post a pic of this cartridge next to a .223 and maybe a .300/221?

Jon


no problem......

L to R....300 Whisper, 7.62x40mm, .25x40mm, .223

497

[kurtz]

.25x40mm Range Results

505

Load Data: (standard disclaimer as with any wild-cat, use at own risk and work up slow)

.25x40mm 16" 1:10 carbine length gas system, fire-formed brass...worked up several loads between but only posted the whole unit loads....

100 SGK 19gr 1680 = 2,310 fps
100 SGK 20gr 1680 = 2,411 fps
100 SGK 21gr 1680 = 2,520 fps

100 SGK 19gr H4227 = 2,530 fps

506

kurtz

[kurtz]

Range results:

Load Data: (standard disclaimer as with any wild-cat, use at own risk and work up slow)

.25x40mm 16" 1:10 carbine length gas system, fire-formed brass...worked up several loads between but only posted the whole and half unit loads....

85gr Nosler BT, 21.5gr 1680 = 2,757 fps
85gr Nosler BT, 22gr 1680 = 2,810 fps
85gr Nosler BT, 22.5gr 1680 = 2,853 fps
85gr Nosler BT, 23gr 1680 = 2,913 fps

Averages of 20 rd. strings......

the 1680 produced very linear velocity results with the 85s.......very good velocity per grain of powder......

I tried about eight different powders with the 85 and 100 grainers today.....the only thing that even came close to the performance of the 1680 was 5744 but ran out of room at 21.5grains and was still 150fps slower....

Kurtz

507

[JFettig]

I've been watching carefully! Those are some pretty dang good velocities, almost 1600ft-lbs. Have you made any abnormal pressure observations? What is the taper like on these cases? They look pretty straight walled.

Jon

[kurtz]

I've been watching carefully! Those are some pretty dang good velocities, almost 1600ft-lbs. Have you made any abnormal pressure observations? What is the taper like on these cases? They look pretty straight walled.

Jon

Hi Jon,

I tried about ten different powders and the pressure indicators ranged from none to destroyed brass, nothing too un-expected, the bad end was ejector and extractor gouges if I got too carried away.....the Accurate 1680 looks to be the go to powder for a 16" barrel, none of the other powders tested came close.....

one interesting observation using Winchester primers: out of 20 rounds fired at 2,900 fps 19 primers looked pretty normal; flat spot in center, rounded edges, slight lifting of firing pin strike........the one odd-ball was so flat that you couldn't have done any better with a hammer and anvil, the brass looked good and the pocket was still tight....thin primer?

the case taper is along the lines of a .308 Winchester....comes out of PMAGS really slick and cycles great...

kurtz