In the recent thread on the Satterlee, Audette, and OCW methods of load development and how I believe they are all related to barrel harmonics, the question has come up whether installing a suppressor increases back pressure or not and whether this cause a reduction in velocity.
I believe that the answer is that it can go either way and it all depends.
Here's some discussion and why:
My mental model of the pressure profile in the can is that as the bullet pushes and compresses air and possibly some gas from the explosion may leak by the bullet in the land grooves, into the can and it creates some back pressure vs. just going out the barrel into atmospheric 14.7 psig.
The pressure in the can especially in a longer bbl. has to be a bit higher than 14.7 psig atmospheric pressure. It just has to be mechanically, therefore it is exerting some amount of back pressure on the bullet before exit.
If you put a filter on the end of s garden hose what happens?
If you put mufflers on your car with baffles to silence the exhaust, what happens to your horsepower? Its backpressure. Thats why they use headers at the race track with no mufflers!!!
Flow rate slows due to back pressure. So I have no idea whether we can measure backpressure due to a can or how much it may slow muzzle velocity but I think it does.
The only caveat I can think of is if you have a long bbl and a very slow powder, then the pressure curve or burn rate curve may have time to more fully develop giving added force from the chamber side of the explosion and increasing velocity that way. It would be similar to deeper bullet seating w crimps on a slow powder and long bbl.
So, perhaps there are boundary conditions where it can go both ways?
I do think it may be hard to measure the effects unless your ammo has a very low ES, say less than 5 fps.
IN short bbls. with fast powders, a can is maybe more likely to slow the bullet because there is no time for the burn curve to do anything more than its already doing....Burn curve = proxy for pressure curve or profile in the bbl.
In a long bbl w slow powder, maybe the velocity can increase because the back pressure delays bullet exit time in milli seconds that allows for a more complete powder burn to occur equals more force on chamber side of the bullet?
In Rifle Accuracy Facts the author has pictures of muzzle blast, pages (Photos of bullet exit p. 135, 136, 137)
He says Now we know that the muzzle blast pressure drops from 5000 psi to about 150 psi in 50 psec for the 6BR but we don't know just how the pressure drops off. So, there is pressure building in front of the bullet as I have been thinking. And though I have thought about burn rates with different pressure profiles in different length barrels, I did not realize a faster powder gives a lower muzzle pressure at exit as explained below.
He also says:
The fact that a fast burning powder will result in a lower pressure at the muzzle than a slow burning powder was mentioned earlier. However, for the same muzzle velocity the faster burning powder will produce a higher chamber pressure. Nothing ever comes free in this business! I decided to test this contention by measuring the in-bore pressure at the muzzle using the strain gage method that we used to measure chamber pressure back in Chapter 2.
Since we are only interested in the comparison of muzzle pressures resulting from the two different powders, we don't have to go through the tedious calibration procedure used in measuring chamber pressure.
P146
So, the use of different burn rate powders and the length of the barrel may in fact have some bearing on what velocity we will see with the use of a suppressor and whether it goes up, or down or stays the same?
The author also drilled holes in a barrel and made it vented at the end, a muzzle brake of sorts. Here's what he found.
It reduced muzzle velocity.
He does not explain why. But, if gas that could be used to propel the bullet is siphoned off, it makes sense I suppose.
A can is more confining than a muzzle brake of gas and more or less traps and vents it off after the bullet is gone. But, as I have stated above,
perhaps it can go either way with a can.......hard to say.
More quotes"
Well OK, did the muzzle ventilation help? It did. It reduced the average 5-
shot 100 yard group size from about 0.35" to 0.23" with match bullets and a
14" twist in the 6mm Remington rail gun. This is not as good as a top flight
6PPC HV gun or the 6BR rail gun which average in the high ones (i.e., 0.18").
The ventilated muzzle might be a good idea on sporters and long range magnum rifles, but there is talk about outlawing it in Hunter Class bench rest competition because of the increased muzzle blast on nearby shooters.
The muzzle blast pressure can be reduced by using a smaller grain or faster burning powder, longer barrels, lower chamber pressure, or a muzzle ventilator. How- ever, for a given case volume
you have to give up velocity, or bullet weight,
or increase the maximum chamber pressure to obtain the same velocity. This usually means that one has to use a lighter load with the faster burning pow- der that won't fill the case, which may result in greater shot-to-shot velocity variations.
P 151
The muzzle blast pressure was measured using strain gages and it was deter-
mined that large cases (270 Win.) with relatively slow burning, large grained powder had a much larger muzzle blast pressure than relatively fast burning small grained powder. Muzzle blast pressure was also decreased with longer
barrels and ventilated muzzles. Muzzle ventilation was tried and it did reduce the muzzle blast pressure as expected and produced a significant reduc-
tion in group size. However, the type of muzzle venting that was used is a
difficult machining job that would be expensive to do in production.
Now from other sources I searched for:
This below says suppressors cause back pressure, although newer designs cause much less than what they once did. How and when it occurs is critical.
Most have little effect on bolt guns beyond heat transfer and a sticky bolt that is hard to lift. Gas guns are a different story. Increased back pressure causes increased bolt speed and can wreak havoc on function. Piston-driven systems are less susceptible, but can still be problematic. As a general rule, the shorter the barrel, the greater the effect on the gun's performance. Adjustable gas blocks help but remain an issue. Excess gas in the action can also affect reliability as the action can get fouled quickly. Rapid fire produces significant heat transfer to the weapon and can have an adverse effect on operation.
https://gundigest.com/gear-ammo/suppressor-effects
Might go either way depending on baffle design and where gas goes....
https://www.snipershide.com/shooting/threads/backpressure-and-suppressor-design.6938437/
No statistically significant impact.
https://www.thetruthaboutguns.com/ask-foghorn-does-a-silencer-effect-the-velocity-of-the-bullet/
Maybe we will learn something from this?
I believe now it can go either way depending on suppressor design, rifle barrel length, and powder burn rate, and your ability to measure
slight variations in velocity FPS within the quality of the ES and SD of the Ammo you are firing.
