If barrel length and weight didn't matter

[P95Carry]

I'm sure this is not a new thought process .. and confess I haven't searched to see what has been discussed before.

Is approx 24" and thereabouts an optimum length of barrel (let's loosely stay withing the 30 cal area)? I guess it is. I only ask because IF we had a barrel near twice as long (forget for now probs re weight, support etc) ... and IF we had stronger chambering ..... and IF we had a slower powder in a larger case ... then could we expect that the bullet's acceleration phase could be extended that much ... so we have a very marked increase in velocity potential. Thence longer range and energy levels.

Acceleration obviously ceases at the muzzle .. it's all downhill from there! But could we extend a gas burn long enough ... and get enough volume of gas .... to maintain acceleration that long/far?

I am tempted by thoughts of barrel resonance problems .. decreasing accuracy .... but in principle .... what are the pro's and cons ... again leaving aside ergonomic factors.

Rifle guru's ... what say you? .... and sorry if this is an old subject.

 

[Dionysusigma]

Bump... because I've wondered this too.

 

[Jim K]

The best way to ensure good groups at 100 yards would be to have a 100 yard barrel. A bit awkward in the woods, though.

Given all those givens, high velocity should be possible. You could even use the German "channel gun" principle and build additional powder chambers into the barrel at intervals. Or use a rocket with the barrel only as a guide, like an enlarged Gyro-Jet, which would not stop accelerating at the muzzle. But electro-magnetic propulsion seems a better way to go if size and weight are no consideration.

Jim

 

[lycanthrope]

Barrel weight and length DO matter....depending on the cartridge. To get speed without high pressure you need to build it slowly and evenly. This usually means a slower powder which necessitates a longer barrel. My 7mm STW needs are least 27" to really get humming (120gr at 3700fps with IMR 7828). With that much powder, having a heavy barrel helps with heat reduction which can kill these bores.

I'm sure 24" isn't optimum for a 300 Ultra mag.

At some point, however, you reach the max for a certain cartridge and you can't get any more pressure without stressing components or detonation of the the propellent (detonation in gas engines can blow holes in pistons so imagine what smokeless powder can do!).

 

[shoobe01]

The pressure peak will not continue climbing to infinity with an arbitrarily long barrel. There is limited propellant, limited oxidizer and very non-zero friction. After a while, the bullet will begin to /decelerate/. This happens faster than you might think. For example, the .45 ACP is going slower out of a 16" barrel than out of 8" (I don't know where the crossover is). Acceleration even in a 24" rifle may not be continuing till it exits the barrel. Often it has started to slow down very fractionally, but this is traded for accuracy.

In theory, for optimal accuracy you would want all the powder to burn off, and toss in a few inches for safety. This way the pressure when the bullet leaves the muzzle has stabilized and is even when it broaches the muzzle.

In practice, this seems to matter less than it should. The pressure curve peaks almost immediately and then drops off quite rapidly. Only the last 10-20% of area under the curve is in the last 3/4 of the barrel. So allowing the bullet to leave the barrel in this phase is usually fine, as its considered coasting.

At some point, minor accuracy differences achieved by extra barrel are overcome by other issues, such as whip and compession (vibration). Some AR-15 shooters especially have gotten very good long range results with relatively short and very fat barrels. These reduce the ability to move, which is more important than the extra velocity. Yes, they also load somewhat faster burning powders, in general.

A pump gun (the german experiments Jim refers to) is tricky to engineer, especially with multi-stage boosters. Accuracy would be low. And erosion occurs across from each chamber, so the barrel becomes increasingly inconsistent. Rockets are typically not so accurate either, but they do accelerate all the way to burnout. Electrics are probably the wave of the future. Railguns and coilguns can now throw projectiles over 4 km/s (13,000 fps) but still generally require inconveniently large buildings (well, trailers now) to power them. Someday...

 

[rdbrowning]

Some of the long range shooters are shooting 30 - 36" barrels to get maximum velocity to reduce wind effects. Velocity isn't so important for trajectory because you can adjust for it, what with gravity being so predictable. Wind, however, is hard to read, inconsistant and really plays havoc witth your point of impact. Therefore you tend to want to use the heavest bullet for caliber with the highest BC available at the highest velocity that your gun can shoot accurately. But there is a point of dimenishing returns and there can be a considerable accuracy problem from whip. Dan Lilja has a good article about a test he conducted starting with a 46" barrel and doing velocity testing as he progressivly made it shorter.

http://www.riflebarrels.com/articles/bullets_ballastics/long_barrel_velocity.htm

 

[Frohickey]

Um... if you are trying to kill something at 100 yards, and you have a 100yard barrel, isn't it better to just have a 100 yard knife instead?

 

[lycanthrope]

No...because knives don't don't displace tissue from shock.....

 

[P95Carry]

rdbrowning .... many thx for that - excellent and fascinating excercize ... great reading. Not only impressive data but I doubt many people would have carried out this sort of experiment.

Quite some round eh? Near 2,000 ft lbs of energy still there at 1,000 yds .... awesome.

So .. does seem pointless getting too long ...... rather as I suspected, barrel whip/resonance is more than possible. Then too the relatively small bonus in velocity.