Barrel length vs volocity

[ped]

i read somewhere that 3" of barrel is all that is needed to achieve enough spin for accuracy
ped

[BDS]

i read somewhere that 3" of barrel is all that is needed to achieve enough spin for accuracy
ped

Well... don't know what the right number is but, FX air rifles have the Smooth Twist barrel which is just that, 10 inches of smooth bore and the last few inches rifled.

Freddy Axelson (FX) is a clever guy so I'm guessing it works?

[Flex]

As I've said on more than one occasion, I'm neither a firearms engineer nor an air gun expert but these forums (be it good or bad) allow any member to state their opinion as long as it's done in a civil manner.

As stated by many of the replies so far, barrel length is only one of several factors that contribute to pellet velocity. Pellet weight is certainly a factor, as is valve output and whether you're on the first shot of a CO2 cart or near the last.

RE: barrel length, friction is definitely a factor. Assuming the same condition of the bore, the effect of barrel friction on a pellet is less with a shorter barrel than with a longer barrel (That's probably pretty obvious to most folks). However, I don't think that friction becomes a major factor until the barrel length becomes excessive. In between the extremes of too short and too long, the barrel's length has it's effect on velocity by allowing the expanding gas from the CO2 cart to continue to accelerate the pellet until it exits the muzzle.

RE: pellet weight, basic physics (Newton's 2nd Law of Motion F=ma) tells us that, with other factors being equal, when the same force (F) i.e. CO2 valve output, is applied to a lighter pellet (m), the acceleration (a) will be greater.

RE: valve output. For many of us this will be a constant as no attempt will be made to "mod" the valve to increase it's output. Many, of course, will apply a great number of mods to increase valve output. Lighter internal springs, heavier hammer springs, porting and polishing the gas pathway are among the most common.

RE: CO2 cartridges, they are, for the most part, a constant also. The CO2 under pressure (~800 psi) in the cartridges exists as a liquid. This allows the output pressure to remain constant until the CO2 liquid is depleted.  This, of course, makes CO2 a perfect gas for this purpose. Gases that liquefy at much higher pressures (and / or lower temps) like air, oxygen and nitrogen are not suitable in this situation (i.e. gas filled cartridges).

Anyway, back to the barrel length issue. Assuming standard CO2 power source, optimum barrel length would be that where the valve output is no longer accelerating the pellet at the point that it leaves the barrel. Easy to say, not so easy figure out. Especially if a variety of different weight pellets are used. Basically, it's a trial and error process. Fortunately, most of the leg work has already been done! Most of the searching that I did indicated that for the 2240 basic air gun, an 18" would be an "optimum" length. That is what I have on my 2240 .177cal hybrid. I've yet to run it over my chrony so hard data is yet to come.

Opinions are worth what you pay for them and since this is a free forum, let the reader beware . I'm just sayin'.
Flex

[BillK]

Six mo. ago I got a 2250XE w/24" barrel.  It was a good barrel @ 5 meters or so but at 20 meters it was all over the place.  So I changed it for an 18".  I prefer that length anyway.  The 18" is dead on at 100'.  This isn't about accuracy, just letting you know why the change.
I also got one of Davios med. hot, not full blown boss type valves.
With the PA turned all the way in the 24" shot a high of 653fps w/CPHP.  10 shot avg. 649fps.
Did the same after changing barrels, 18"    "    "   "    "   590fps "   "          "    "     "    582fps.
Your results may vary.
2250XE w/18" barrel

[airriflenut]

BillK mentioned my earlier test in .177 bulk-feed from a 12 ounce CO2 GMT with an 18" and 24" barrel.  There seas a velocity drop with the 18" barrel, I don't have my chrony data handy but there was a velocity drop.

I've just built a new gun on the Disco tube and wanted to compare the 18" and 24" on bulk-fill.  I lost almost 100 fps with the shorter barrel.  24" average 843, 18" average756.