This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.
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Why bother with the far-end, gain-twist rifling? Spin energy is well less than 1% of muzzle energy: negligible savings and incredible loads on driving bands. Heavy gilding copper may not take your loading, lexan will not, sintered/fused iron is a maybe/maybe not. Small-bore, long barrel, great energy: a large portion of it in the 60-70 grains of gas following the projectile out the muzzle. Barrel wear might be a concern.
Type in 308 smoothbore 5000fps, and theres a vid of a guy who achieved somethig somilar...i cant load it so didnt see the bullet weight.
Also theres a russian smoothbore gun TS2, with paradox rifling.....whats that
Lastly ARL apparently got 5000fps out of a taperbore AR, with 100k PSI
Could one have he full twist rate only on the last 6 inches of barrel....or is that not enough.
You seem to think that using a smooth bore is a necessary condition to achieve extremely high velocities. This is not so. The tank guns started using smoothbores because of the necessity to launch extremely long penetrators. For reasons of gyro physics, these long projectiles simply cannot be stabilized by spin. That is why smoothbore came into play.
So far, nobody has been able to achieve with smoothbore small arms a target dispersion comparable to that of rifled small arms. Five to ten times larger small arms dispersion is typical.
If you would do even the most simple and trivial research before posting questions, you would have long found out that paradox is a very old way of trying to improve slug dispersion from shotgun barrels. Tried hundreds of times, never a success.
Your proposal of starting with a bore rifling of 0 degrees which increases to the necessary rate of twist has been in daily routine use for decades, for example in the German 27 mm aircraft cannon of the Tornado and other aircraft. Starting the rifling not inf front of the chamber but some distance down the barrel (hopefully with the projectile already very fast) would introduce a BIG transient in the entire physics of barrel movement. In my view, it is a recipe for BIG dispersion.
The general rule in armament is that our modern smart ideas of improving are never new. They have been tried already a century or two ago. In some cases, available technology and materials of the time prevented success. But the mechanically unsound like paradox are today as useless as they were long ago.
Yes i should have clarified the idea was really for an infantry round, not an MG round so the dispersion should be fine with single shots. Again, its well proven that slower twist rates or no twist rates, will boost speed. So that was the logic...a 28 inch gain twist barrel to get 4000fps with a 6mm bullet.
Back with the GDs movig barrel...do you think its an accurate gun or should hey have taken the FG42 approach
Quick favour...i cant load the video (or forum article on snipershide.com) about the 5000fps 308. Could anyone tell me the gist of it...how it worked
I don't see the long smooth-bore section of the barrel giving you a velocity advantage (one-percent is conceivable). Concur with JPeelen's points. A very late and short (about a single turn) rifling to achieve the desired spin rate for stabilization is problematic: extreme forces acting on projectile body/driving bands. Engraved portion would have to be very substantial to maintain integrity during late spin up. I imagine it would be quite akin to a squeeze-bore system. Barrel dynamics would be strange. A short freebore before rifling might have some advantage, but I don't see how. Establishing the rifling lands and grooves soon after chamber exit gives the projectile a longer shaping period and minimizes working stresses on the grooved shell/driving band. Progressive twist rate is also problematic for much the same reasons.
Well, Roy Weatherby found some freebore helped reduce pressure on his admittedly hotter rounds. He used way less than SMG is proposing, though.
For someone who wants to duplicate .257 Weatherby ballistics, some freebore makes sense, but I'm still failing to see the military objective.
A .257" / 90 VLD launched at 4000 fps from a 29" barrel is probably a solution, but I don't see the problem the OP wants to solve (the M948 SLAP is probably what you want).
Im leaning away from the initial smoothbore and going with a twist that starts extremely slowly. Only last 5 inches has thr full rate
Another worry is the 90grainer comig apart...at 4000fps. Especially as i planned a hollow aluminium bullet surrounding a 35mm steel rod. Like a mini APFSDS.
The most impressive 6mm rpund is the 244 H&H. A 90grain with 3500ft lbs was handloaded acording to wiki.
The logic was to streamline that into a bullpup battle rifle. Wih an FG42 buffer system.
Sabots leave a mess and arent ideal. A sabotless flechette with fins that ride the bore....like the old russian tank round...
If you want this to be an infantry rifle with a 29" barrel you have somewhere between 4 and 7 Inches behind the chamber In which to build your guns action.
Something over 33-36 inches long is flat unacceptable for general issue infantry use.
You can also get 4000 fps in quite a bit less barrel multiple different ways.
Also your projectile construction is going to cause you severe issues.
What you're trying to emulate is essentially an APCR round (armor piercing composite rigid) quite a few ww2 high velocity tank gun and etc rounds did a similar thing to what you're wanting to do.
You should maybe look into exactly how those APCR rounds were constructed and see if you can emulate their construction and etc.
Yes im abandoning the idea and sticking with the saboted .20 cal, or that bore riding dart.
Just to repeat an old question i asked, whats the weight of some of the heavier 556 steel cores, somethig like 40grain? and if one had a hypothetical 20.cal which is entirely steel, could you realistically take it up to 55 or is steel not that dense