Load

I have had house rules for changing between 1:12 ammo and 1:7 ammo for a long time for GURPS.

Previously it had been:

5.56x45mm and .223 Remington weapons are listed with the rifling pitch listed with their ammunition, for example 5.56x45mm 1:12 has a 1:12 barrel. 1:8 is treated as 1:7 and 1:10 is treated as 1:9.
1:7 barrel, M193 Ammo. -1 Acc.

1:7 barrel, NATO, Mk262 or 69gr SMK Ammo. No effect.

1:9 barrel, M193 or 69gr SMK Ammo. No effect.

1:9 barrel, Mk262 Ammo. -2 Acc. If the temperature is 15°F or lower, then halve ranges, -5 Acc.

1:9 barrel, NATO Ammo. No effect unless using tracer ammunition or the temperature is 10°F or lower, then halve ranges, -4 Acc.

1:12 barrel, M193 Ammo. No effect.

1:12 barrel, Mk262 or 69gr SMK Ammo. Quarter ranges, -4 Acc.

1:12 barrel, NATO Ammo. Quarter ranges, -4 Acc, change to pi-.

An additional penalty of -1 to Acc if changing between loads without zeroing the weapon.

Based on this report I think I shall change my rule to:

Ammunition 62gr or heavier will give a -5 penalty in 1:12 barrels. Ammunition 75gr or heavier will give a -3 penalty in 1:9 barrels. There is a -2 penalty to changing ammunition weight without zeroing the weapon.  If the temperature is below 10˚ F add 5gr to the effective weight of the bullet.  -2 if you change loads without rezeroing with the exception of matched ball/tracer combinations; M193 and M196 are identical in 1:12 weapons and M855 and M856 are identical in 1:7 weapons.

M193 ball and M196 tracer are 55gr.  M855 ball and M856 tracer are long for their weight so are treated as 69 and 72 grain respectively.  Mk 262 is 77gr.