Gear Rifles - design assistance requested

[Varyon]

Originally Posted by Anthony

If it's expressed in units of force it's almost certainly in error, the peak instantaneous force for a .223 rifle is in excess of 2,000 lb and that's a relatively low recoil rifle. I suspect the sources you were looking at were actually giving foot-pounds, which is a unit of energy.

I think I may have jumbled some of the sources I was checking together; the less-reliable ones talked about pounds, but the one I was remembering with the note of most service rifles being around 15 was indeed talking about foot-pounds, which simplifies matters. Rereading that one, I'm considering that perhaps the maximum recoil before you start suffering penalties* is more around 3/4*BL in ft-lb for a rifle (so 15 ft-lb for a character with ST 10 and thus BL 20). I'm fairly confident this is going to mean the weight of the weapon will be the limiting factor for most gear rifles - I think I worked out my reference weapon would be roughly 5 lbs (assuming twice the weight of a comparable firearm), so 251 J - 185 ft-lb - in a 0.1 lb bullet corresponds to a mere 3.7 ft-lb recoil for a 5 lb rifle.
(EDIT: Actually, now that I look at it, if I use BL for weight and 3/4*BL for recoil, those two call for the same ST - ST 5 gives BL 5, and 3/4 of that is 3.75)

It seems like the gear rifle would have an odd recoil profile, although I could be mistaken, but I don't think this would really have a detrimental effect (should just be part of Familiarity). Basically, you get traditional rearward recoil from the piston being pushed forward and the bullet being propelled, but you'd also have some forward recoil when the piston gets arrested upon striking. For most rifles, I think the piston slamming into its base is probably the noisiest part of "firing" the weapon - although my spreadsheets do indicate that pi- (5 mm) rifles break the sound barrier at around ST 17, pi (10 mm) rifles do so around ST 27, pi+ (15 mm) rifles do so around ST 37, and pi++ (20 mm) rifles do so around ST 47. This would technically require the compressed air to get rather hot, so that the speed of sound in the air increases... but this is a modified fire piston, so I'm alright with that, even if it isn't strictly realistic.

*Realistically, above a certain threshold, I think it's more that you can manage some number of shots before the soreness starts damaging your accuracy, but for GURPS purposes, a set cut-off works best.