Rate of Fire Questions in regard to barrels and suppressors

[Haris1]

Hello All:
In regard to pistol caliber subguns
1. Which will have a higher rate of fire: A longer or a shorter barrel on the same gun?

2, Will a suppressor increase or decrease rate of fire?

Thanks

 

[strobro32]

1. longer

2. increase

 

[Haris1]

Strobo
I agree that suppressed will increase
but on a longer barrel, since the dwell time and the bullet is in the barrel longer
why does the longer barrel speed the rpm up?

 

[A&S Conversions]

Strobo
I agree that suppressed will increase
but on a longer barrel, since the dwell time and the bullet is in the barrel longer
why does the longer barrel speed the rpm up?

The longer barrel (10 to 11 inches) has more complete burn of the powder. Until the bullet leaves the barrel, the high pressure force is continually being applied to the bolt. The longer the force is applied, the greater amount of energy is transferred into the bolt. The greater the energy in the bolt the greater the energy put into the recoil spring. Additional energy beyond the recoil spring would be the bolt hitting the buffer and end plate of the receiver and recoiling back towards the barrel. So the cycle of the bolt would be quicker, increasing the ROF. It is the same reason that a can tends to increase the ROF. The longer force is applied to the blowback bolt the more force is in the bolt. This additional force increases ROF.

Scott

 

[Deerhurst]

Pretty sure I'm thinking the same as Scott here.


Longer time of high pressure in the barrel translates to faster bolt velocities which is higher cyclical rate. I know for a fact mac runs faster with a can though it is simple blowback.

 

[mattnh]

Yes - Anything that increases back pressure time will increase ROF (eg longer barrel, suppressor)

 

[SecondAmend]

With respect to barrel length, when I tested .22LR in an M16 as I went from a 4.5 in. barrel to a 9 in. barrel, the ROF went up, and then with a 16 in. barrel the ROF dropped down but not as low as to the 4.5 in. barrel ROF. I speculate that .45 ACP could well exhibit similar performance. Other caliber ammo?
With respect to silencer effect, I couldn't get the timer to reliably capture suppressed fire, but from what I could hear, the suppressor I have for 9mm (Uzi tested) and .380 ACP (M11-380 tested) has no discernible effect on ROF. That said, the suppressor is a subgun suppressor, so it is somewhat long and has a fairly high volume. I don't believe that the particular suppressor produced much, if any, appreciable back pressure. A "tighter" silencer would likely have a rate increasing effect.
MHO, YMMV, etc. Be well.

 

[A&S Conversions]

It is my understanding that typically pistol caliber rounds tend to have most of their powder burned somewhere around 10" of barrel length. A shorter barrel and there will be unburned powder when the bullet leaves the barrel so the round will not reach the highest speed possible. A longer than optimal barrel will start to slow the bullet from friction between the rifling and the bullet. My cousin chronographs his Beretta 92 and a 16" barreled Beretta Storm. They were within an average of 5 ft/sec using the same ammo.

I can't speak to anyone else's ear, but I need a PACT timer to tell the difference between 1,300 and 1,400 RPM. I have had better luck putting the timer near the muzzle to capture suppressed ROF. YMMV.

Scott

 

[heimue]

I understand the principle of --> bullet is in the barrel longer --> higher pressure --> higher force on the bolt --> faster rpm. That's the easy part.
But - on a simple blow back operation, does it even apply? By the time of when the bullet leaves the barrel, where exactly is the empty shell? The bolt will not get any more force/acceleration as soon as the case is out of the chamber. Wherever the bullet is at this moment must be the point of where the barrel length doesn't make any difference anymore. Anybody has the information of bullet speed vs. case speed?

Dieter

 

[Deerhurst]

It is my understanding that typically pistol caliber rounds tend to have most of their powder burned somewhere around 10" of barrel length. A shorter barrel and there will be unburned powder when the bullet leaves the barrel so the round will not reach the highest speed possible. A longer than optimal barrel will start to slow the bullet from friction between the rifling and the bullet. My cousin chronographs his Beretta 92 and a 16" barreled Beretta Storm. They were within an average of 5 ft/sec using the same ammo.

I can't speak to anyone else's ear, but I need a PACT timer to tell the difference between 1,300 and 1,400 RPM. I have had better luck putting the timer near the muzzle to capture suppressed ROF. YMMV.

Scott

I have a CED-7000. Il.have to see if I can capture ROF changes with it. I know it does catch suppressed shots very well. It'll even catch dry fire most of the time in normal mode.

You are correct on the barrel length vs burn rate as far as I have found. When I chopped my 7.62x25 AR barrel down to 8" from 16" my muzzle velocity went up a little. Powder is all burned in 8-10" depending on the powder I use. I had to take that I to account when I loaded subs for it.

 

[Haris1]

rate of fire questions

This is great information
Thanks for all the great comments

 

[A&S Conversions]

I understand the principle of --> bullet is in the barrel longer --> higher pressure --> higher force on the bolt --> faster rpm. That's the easy part.
But - on a simple blow back operation, does it even apply? By the time of when the bullet leaves the barrel, where exactly is the empty shell? The bolt will not get any more force/acceleration as soon as the case is out of the chamber. Wherever the bullet is at this moment must be the point of where the barrel length doesn't make any difference anymore. Anybody has the information of bullet speed vs. case speed?

Dieter

With a simple blowback system the mass of of the bolt and the force of the recoil spring needs to hold the shell in the chamber during the high pressure phase (the time that the bullet is in the barrel while the gas from the burning gunpowder is acting on it). The pressure drops when the bullet leaves the barrel. The mass of the bolt is just starting to cycle back. If the bolt doesn't have enough mass, the shell will push the bolt back while the bullet is still in thebarrel. The shell is base is fairly thick, but thins quite quickly. With tens of thousands of pounds per square inch pressure inside the shell, without the chamber of the barrel supporting the side wall of the shell, the shell will rupture. If you are very unlucky, the shell wall facing the port door will rupture spraying scards of brass at high velocity out the port door.

When the bullet leaves the barrel it is traveling hundreds of feet per second. The barrel is 6", 10" or 16". How many milliseconds would the bullet take to cover those distances? On the other hand, how fast is the bolt traveling? It isn't any where near as fast as the bullet. I don't have the specific bolt velocity but with "equal and opposite reaction", the 147 grain bullet going around 1,000 ft/sec and the pound bolt with a recoil spring force resisting it would be exponentially slower. So the bullet would be out of barrel long before the bolt got very far.

Scott