Why does a suppressor on a 50 BMG increase felt recoil ?

[big5ifty]

I was chatting to Des the other day about reducing the recoil of a BMG rifle by replacing the brake with a Sus-Tac suppressor.

Des said the suppressor increases felt recoil.

I was skeptical, so I went through a bunch of Youtube videos, and found one that supports that opinion.

You can see in the video that the with the brake, the bipod feet stay on the ground, and with the suppressor, the bipod feet bounce from the recoil.

This indicates there is more recoil with a suppressor than with the brake.

Why is that happening on the BMG, but not on smaller calibers ?

[Treeman]

One of those things that will now mind hammer me till I know.  I would hazard that the amount of gas vented from the suppressor actually gains the effect of rocket propulsion in recoil direction. This would likely be based on a accumulative bases where it only comes into effect after certain volume is reached. 

[big5ifty]

I spoke to Martin from SusTac, and he confirmed that there is a percentage increase in felt recoil with their BMG suppressor.

It remains to be seen in the bit of extra felt recoil is a good trade-off to eliminate most of the muzzle blast.

[oafpatroll]

Are the ports of the brake angled backwards? If so I'd expect that they would contribute to reducing recoil given how much gas a 50 produces. Was at SWAT a few years back when a guy arrived and started shooting with a Steyr similar to that with a honking great brake. It was bold, capitalised and underlined OBNOXIOUS.

[Treeman]

Told you this would mind **** me till I figured it out.

The Suppressor does not increase felt recoil.
The suppressor has more recoil than the brake yes, but not more than the bare barrel would have.

So if you shot it bare barrel, then suppressor, then break you would say recoil decreased in same order. If you shot it with a brake and then with an suppressor, you would believe recoil had increased.

[223]

As Treeman said above:

On a normal (un-braked) barrel, the gasses vent straight forward and at quite some speed.  You can calculate the momentum of the escaping gasses Momentum = Mass x Velocity.  Mass of gas is the mass of the powder.  Average gas velocity is about 6000 fps.  (This could vary, but is quite hard to measure.)  Normally the momentum of the gas is very close to the mullet's momentum, so it makes up about half of the total recoil momentum.

Forward momentum (bullet +gas) = rearward momentum (rifle).  (Ref: Isaac Newton.)

If you add a suppressor that vents only straight forward, the mass of the gas stays the same, but the velocity is reduced to about 1000 fps or less.  This reduces the recoil generated by the gas to a small amount.  The weight added to the rifle by the suppressor also helps to reduce recoil velocity.  The rifle's recoil momentum stays the same, but it is transferred to the shooter over a longer period of time.

If you have a brake with 90 degree vents, the gas direction is turned sideways, removing all momentum contributed by the gas.  So this cuts the recoil by about half.

On most 50BMG rifles, the ports in the huge brakes (not breaks, please?) are angled backwards.  This setup actually uses the escaping gasses to "pull the rifle forward" a little.  The recoil momentum is now the bullet's momentum LESS the backward component of the escaping gasses' recoil.  The total recoil is thus less than with any of the above.  On a .50 this is needed.

There is a trick to suppress them and still keep the recoil manageable, and that is to add sideways brake vents to the front of the suppressor.  While the gas velocity is pretty low, changing the angle still makes a significant difference, as there is still about 220gn (14g) of gas flowing out.  With the help of the extra weight (not very light on a 50) this seems to give very similar perceived recoil to a proper brake, without all the blast and dust.

Obviously, the same goes for all other calibers, but with our "normal" hunting and target rifles being much more complacent, these differences are far less noticeable.

Part of the fun of owning or shooting a 50BMG rifle is the fact that everyone notices that it has gone off.  Someone once said it feels as if you are presiding over your own private earthquake.  So keep the suppressors off and just triple plug your ears...  And all your other orifices...