So I did a bit of tinkering. I can get a 35grain projectile to like 3850 ft/s v0, but the ballistics are so abysmal that at 100yds it is 3150 ft/s and 2850 ft/s at 150 yds.

good enough for a PDW I'd say?

That's actually not horrible. Since you're going to be limited by bullet length pretty hard you may be forced to accept pretty hellacious velocity drops out at range.

But with a pdw you're mostly trying to be a credible threat past 100 not an actual threat.

That gives you some latitude.

Also, that monster 3850 up close is going to make for something pretty killy even with kinda lame but cheap projectile construction.

so we have the gun. I haven't done the twiddling of swept bore vs external ballistics again, but will guesstimate at something like .25.

So we have the gun and ammo. The first 200 are delivered to the Elbonian Army for first trials. What happens?

oh, COAL is 40mm, case length is 25mm, base diametre something like 12.5 mm, magazine size 35

 or so.

roguetechie said:

That's actually not horrible. Since you're going to be limited by bullet length pretty hard you may be forced to accept pretty hellacious velocity drops out at range.

Bullet length is 20mm. I need to make it this light to get the speed. So a hardened steel core and polymer for the rest of the bullet body.

I think it is worth revisiting rate of fire. Sorry just watched one of Ian's videos ;) https://youtu.be/FAIyQ5yqVu8 And it seems that either slow or fast are sweet spots. And given that this gun has some recoil, I think slow is the way to go. 450 RPM?

Also, how likely are people to use it in some kind of full auto? Badly trained REMFs being surprised?

I am inclined to vote for a fast cyclic rate with 2-rd (or perhaps 3-rd) burst limiter:  https://youtu.be/gcIKXqZyEOI?t=1

Low versus high cyclic rate SMG comparison test (Grease gun vs Kriss Vector):  https://youtu.be/1HlAUsWaahU?t=22

stancrist said:

Low versus high cyclic rate SMG comparison test (Grease gun vs Kriss Vector):  https://youtu.be/1HlAUsWaahU?t=22

interesting point. constant recoil would make the gun too complicated I assume?

graylion said:

I think it is worth revisiting rate of fire. Sorry just watched one of Ian's videos ;) https://youtu.be/FAIyQ5yqVu8 And it seems that either slow or fast are sweet spots. And given that this gun has some recoil, I think slow is the way to go. 450 RPM?

Due to muscular reflex, the stability of full-auto fire is like a "U shape" as described by Ian.

At low rate of fire (below ~10 Hz or 600 rpm), the muscular response is in the same phase as the impulse, so the shooter can control the shot-to-shot muzzle jump.

Between ~10 Hz  and ~16 Hz, the muscular response become more and more out of phase from the impulse and the dispersion dramatically increases, ~16 Hz being the worst possible RPM for full-auto stability.

Between ~16 Hz and ~20 Hz (1200 rpm), the muscular response can't follow the impulse and the dispersion starts to decrease as the recoil feels more and more as a continuous "push", and not discrete pulses.

Above 20 Hz, there is no longer parasitic muscular movement and the dispersion is a linear function of the impulse.

Interesting. In the Shrivenham small-arms museum there is an SA80 modified to have an electronic trigger, which can be set to any desired rate of fire. As I recall (I was told about it, the experiments took place before I had ever visited) the optimum rate of fire for controllability in auto was found to be 300 rpm.

autogun said:

Interesting. In the Shrivenham small-arms museum there is an SA80 modified to have an electronic trigger, which can be set to any desired rate of fire. As I recall (I was told about it, the experiments took place before I had ever visited) the optimum rate of fire for controllability in auto was found to be 300 rpm.

Yep, I think that everyone made more or less the same experiments.

That's why we selected a firing rate above 1100 rpm for the FAMAS, only to discover that from an operational point of view, what you need is semi-auto firing for dealing with medium or long range, and full-auto for short range. 3 shot burst consume 3 times more ammo than semi-auto at medium and long range without any gain, and does not produce the same effect that a longer burst at short range.

So the studies made during the '80s for the "follow on" FAMAS MSD focused on the lower bound of the rpm, and found than depending on the weapon configuration and ammo impulse, the best results were achieved with a rate of fire between 350 and 450 rpm.