don't think of barrel length and powder burn when thinking performance or accuracy, think barrel twist. you at least want to spin the bullet 1 complete turn. for example a 1 in 16 means 1 complete rifling "spin" in a length of 16".
the rifling twist helps to spin the bullet, which centrifigul force helps to stabilize the bullet in flight once it leaves the muzzle, just like throwing a football.
if it is a 1 in 16 twist as most .22lr manufacters have, a 16" barrel + chamber length will give you that 1 full complete spin of the bullet.
just like a centerfire, a longer bullet heavier (such as the 60 gr aguila) will work better in a "faster" twist like a 1 in 9 compared to one with a shorter lighter bullet (40 to 29 grains) that a 1 in 16 twist.
a wiki explaination and equations for the best twist rate for any given projectile
Rifling - Wikipedia, the free encyclopedia
Rifling twist rates and bullet accuracy
if you want
to think powder burn, it'll depend on the powder charge / FPS of the ammo used. again, most powder in high velocity will be burnt up in that 17 to 20 inch range, standard velocity / subsonic / match ammo will burn up in the 14" to 18" range.
i cut a 20" barrel down to 16" and chronied both HV and match velocity ammo. at 19" there was a noticable drop in FPS, but after that the amount of FPS was barely measurable. i've further cut it down to 16", and same FPS range as the -1 and -2 inch readings.
from a post elsewhere:
Whether you call it a 20" barrel as advertised, 19.75" measurement from the muzzle to receiver, or 21 1/8" by inserting a cleaning rod into the barrel againt a closed bolt, then marking and measuring the rod, we'll call it the "stock length barrel", -1", and -2".
Rifle used: Savage MKII "F" sporter barrel - 2lb 12oz trigger - shot of harris bipod and rear bag - 12x on the optic. Used 2 ammos, CCI blazer (high velocity) and Aguila Golden Eagle Match rifle. 10 round groups were shot, barrel was dry swabbed "clean" before an ammo change. At 50 yards, 1" bull was used, at 100 yards 1.5" bulls.
Groups with the stock length barrel - weather 65 deg. F., 35% hum., 3-7 mph wind from 8:00, mostly sunny.
Groups with the -1" and -2" - weather 45 deg. F., 47% hum., 3-10 mph wind gusts 3:00.
FPS per length - Chrony was set at 10 ft away from muzzle:
Blazer - 1241 FPS avg., (range of 1221-1256 FPS)
Aguila Match Rifle - 1101 FPS avg., (range of 1080-1126 FPS)
Blazer - 1216 FPS avg., (range of 1207-1221 FPS)
Aguila Match Rifle - 1083 FPS avg., (range of 1062-1095 FPS)
Blazer - 1217 FPS avg., (range of 1173-1239 FPS)
Aguila Match Rifle - 1082 FPS avg., (range of 1062-1111 FPS)
50 Yard groups - stock length top, -1" middle, -2" bottom
100 Yard groups - stock length top, -1" middle, -2" bottom
It appears (in general) that the groups of both the ammos shrank at both 50 & 100 yards. From stock length to -1" there was a 25 FPS drop in the HV Blazer, and 18 FPS in the Match velocity Aguila. When shortened another inch to 2", no significant drop in FPS, but more consistant on paper. Oddly the -1" and -2" barrel length RAISED the POI an average of 1.5" in the HV and remained the same for the match ammo at 50 yards. On the contrary, at 100 yards the POI on the match ammo changed about 1.5" higher - STRANGE!
BULLET DROP - (shooting at 100 yards with the 50 yard setting) the amount of bullet drop changed, but only slightly by about -.75" with the shorter barrel.
1. Shortening the sporter barrel decreases group size with HV and match ammo.
2. FPS was reduced with the initial -1" (around 20 FPS), but the second inch reduction didn't show much more as i would have expected (I figured it would be -40 FPS, but was nearly identical to the -1" FPS).
3. Balance and handling was improved.
So will further shortening the barrel decrease group size more, stay the same, or actually hurt?
After the initial drop in FPS, would FPS stabilize until a more "extreme shortened barrel" length is reached?
Is it that the shorter barrel is "stiffening" up the sporter styled barrel by reducing "whip" and harmonics producing better groups?
I'm not sure at this point, but shortening my sporter barrelled rimfire by 2" actually improved my groups at both 50 and 100 yards, high velocity and match / standard velocity.