Wildcatting and ballistics: 257 Roberts on 7mm-08 case...possibilities?

[HanzMcFeely]

Met an old timer special forces guy awhile back that was talking about having built something like this as his custom built “precision”/sniper service rifle. Curious if it would be a viable cartridge...from what he was telling me, he pushed some odd 3500-4000fps with it and rang steel at 1.5mi (tank silhouette)...

My question is, how can I figure out the ballistics for something like this? Part of me wants to call BS and say he’s full of shit, but stranger things have happened in the wildcat industry...

[Delkal]

The ballistics part is easy........all you need to do is chronograph the velocity and know the ballistic coefficient of the bullet. If he is really getting 4000fps with a .308 case volume and can hit anything at 1.5 miles is the real question.

[HanzMcFeely]

The ballistics part is easy........all you need to do is chronograph the velocity and know the ballistic coefficient of the bullet. If he is really getting 4000fps with a .308 case volume and can hit anything at 1.5 miles is the real question.

That’s what I question also... I could see 3200-3500fps realistically, but 4000?

7mm-08 is a 30-06 case though, isn’t it?

[Delkal]

7mm-08 is a necked down .308 and is shorter than a 30-06. He should easily be able to hit 3200 fps with a light bullet but then the ballistic coefficient will go down. What he is shooting is a slightly necked up .243 so i will call BS on 4000 fps.

[cephas]

Are you saying take a 7mm/08 case and neck it down to .25?
If so its been done before. Old timers used .308 cases to wildcat a 25/08 known as the 25 souper. Remember it being around in my younger days.
"From the usual custom barrel length of 24” (1:10” twist), the .25-08 is capable of firing 75 grain bullets at 3350fps, 87 grain bullets at 3250fps, 100 grain bullets at 3050fps and 120 grain bullets at 2800fps."
https://www.ballisticstudies.com/Kno...25+Souper.html
Also the 7mm-08 is from a .308 case. 7mm with an 06 case was the 7mm-06 commercialized as the .280 remington.

[HanzMcFeely]

Thanks Cephas. Wonder if it was standard 7mm case then instead that he used...

He did mention that recoil was ridiculous and required multiple gas pistons to control and he was hand loading his own ammo. No idea what platform he built on in that case...

What I’m trying to solve for is bullet weight used, powder charge, etc. more of a curiosity than anything else...

[Delkal]

And don't forget the 260 Remington. It is a .308 necked down to 6.5mm (.264 inches) and its performance should be almost identical to the same case loaded with a .257 bullet. The only practical difference between the two cartridges will depend on the availability of different weights of bullets.

[gghbi]

257 is in the same range as 6.5mm. Many 6.5 mm bullets are well known for excellent ballistic coefficients, which is why the 6.5 creedmoor is such an up and comer.

The origin of the 257 Roberts was the 7 X 57 case. There was a 257 Roberts Ackley Improved noted in Wikipedia which lists a 3226 velocity with a 100 grain bullet.

https://en.wikipedia.org/wiki/.257_Roberts

[Gunplummer]

Being a product of Aberdeen Proving Grounds and having worked on guns in two different branches of the military, I am not surprised. There is no end to the nonsense you hear. The favorite in the 60's and early 70's was "the commies made their ammo 3mm longer so they could use ours but we could not use theirs". If it sounds outlandish, it probably is.

[HanzMcFeely]

Being a product of Aberdeen Proving Grounds and having worked on guns in two different branches of the military, I am not surprised. There is no end to the nonsense you hear. The favorite in the 60's and early 70's was "the commies made their ammo 3mm longer so they could use ours but we could not use theirs". If it sounds outlandish, it probably is.

Kind of what I figured...he was probably in service around that time tbh...