SBR Noise

[alamo5000]

Prepare yourself for another one of my nerd alert threads

I have test fired my 10.5 pistol aka soon to be SBR and the thing is basically loud.

I am wondering if anyone knows just how much louder a short barrel is than say a 16" gun and really a basic explanation of 'why' this is so. Basically none if this is trouble shooting or 'need to know' but rather I am try to expand the knowledge base and maybe understand more principles of sound as it relates to guns.

Maybe the Griffin guys will swing by and add their 3 cents.

Why does shortening the barrel add noise? Why does everyone complain about 'that guy'... the one with the brake? Technically wouldn't a compensator be just as loud as a brake? Or is the bitching because the blast is directed out to the side more with a brake?

I am running one of these on my short gun:



So as a baseline I am talking straight up gun noise.

Next I am adding in the use of a suppressor (in my case a Recce 5)...and how much suppression occurs when starting from a higher baseline of noise such as a 10.5 barrel?

I wonder what the actual metering would be on a 10.5 barrel with a Recce 5... vs on a longer barrel? Does the silencer become less efficient with a shorter barrel or is it because the baseline for noise is just that much more? Hopefully that makes sense.

Another thing is someone gave me an ebay gift card for Christmas and really I didn't want anything from ebay...so I shopped around and found one of these:



My thought process for buying that is, yes I will run my silencer on both guns (obviously) but sometimes I might want to run the short gun without one. For those times when I really want to run the gun harder I can use the blast shield... I don't want to damage my suppressor from doing mag dump after mag dump... 3 mags in of running hard and you're pretty much over the heat limit (per a youtube video--which they actually measured the temp of a suppressor from start to finish when running hard)...

I am thinking that little blast shield might 1) look pretty cool on the muzzle, but also 2) direct the concussion out and away from the shooter's position. It will still be loud but it may or may not add something that wasn't there before.

Long story short thanks to the gift card I got a brand new blast shield for almost nothing so I am not really out anything.

But if I am out and we have 3 or 4 people and they all want to shoot the (eventual) SBR I don't want to run my silencer that hard. In those situations we can put up with the overall noise but I wouldn't want to run my suppressor that hard to where it's baking off the finish or damaging the can. Plus without the can on I can take advantage of the shorter OAL of the gun.

Really there are 3 or 4 questions or topics in there so maybe we can get some amateur and hopefully some expert input here.

[alamo5000]

Here is a youtube video that GA made about those blast shields...

[alamo5000]

Also I would like to throw in another question that is pretty technical in nature... if you watch this video at 20:00 in they do the whole heat test on a suppressor...



My other question is it seems like a very short barrel (like a 10.5) will generate a lot more heat in a shorter amount of time for a suppressor.

Is that true and can it be quantified in any kind of relevant numbers?

To me the shorter the barrel the closer you are to the blast and hence more heat...

So the question is about suppressor use on a short barrel... specifically as it relates to heat.

They did that test above in the video... I think it would be cool to do a similar test using a 10.5 and see if there is any heat generation difference based on barrel length...

[Former11B]

Did you eat paint chips as a kid?

[alamo5000]

Did you eat paint chips as a kid?

Dirt. Lots of dirt. LOL!!!

My questions really are brilliant ya'll just haven't realized it yet HAHAAHAAA!!

[SwissyJim]

Did you eat paint chips as a kid?

LEAD paint chips!

I think a lot of it is the powder is still burning while it exits the barrel, hence the more flash/bang. Of course powder selection for reloading can come into play here. Also look at the suppressors info- there are limits to barrel length they are warrantied to.

I was looking at having my Saiga12 turned into a short barrel, and Tony Rumore @ Tromix told me to wear gloves with anything 10½" and shorter...

[alamo5000]

LEAD paint chips!

I think a lot of it is the powder is still burning while it exits the barrel, hence the more flash/bang. Of course powder selection for reloading can come into play here. Also look at the suppressors info- there are limits to barrel length they are warrantied to.

I was looking at having my Saiga12 turned into a short barrel, and Tony Rumore @ Tromix told me to wear gloves with anything 10½" and shorter...

I had frosted lead chips with milk every day for breakfast for my entire 4th grade year

As to powder I have wondered about this too...how powder selection can be different for shorter barrels. Something faster burning might be good. I have some so I might experiment, but I am no Molon. Anything I do would be anecdotal.

I did hear that most powder is already burned within the first so many inches of the barrel but since fluid dynamics isn't really my gig I don't know. Too scientific even for me, but if anyone knows by all means...

My silencer is warrantied down to 7.5" so it has no problem handling a 10.5 barrel... I was really kind of asking that part so I can maintain my suppressor or at least try to. (You have to see the 2nd video to know what I am referring to...)

[SwissyJim]

mmm...frosted lead chips! LOL

I saw a post somewhere, regarding pistol loads, but it showed the difference in muzzle flash between two powders with (supposedly) close burn rates. I'll see if I can find it again. I do know that certain powders make a differnce. Like I have all but given up on 5744 in my 30BLK as I always have a LOT of unburned powder granules in the barrel with it.

[alamo5000]

mmm...frosted lead chips! LOL

I saw a post somewhere, regarding pistol loads, but it showed the difference in muzzle flash between two powders with (supposedly) close burn rates. I'll see if I can find it again. I do know that certain powders make a differnce. Like I have all but given up on 5744 in my 30BLK as I always have a LOT of unburned powder granules in the barrel with it.

I have a 1 pound bottle of IMR 4895 but I am normally shooting Varget.

On this chart the IMR is #89 on the list and Varget is #99.

http://www.lasc.us/BurnRatePrint.htm

It might not make one hill of beans worth of difference but then again when you get down to shooting pistol length barrels I don't know why it wouldn't make a difference.

[alamo5000]

Maybe I stumbled upon a semi answer to one of my questions... Check the chart above...Look at the PSI @ muzzle exit. The shorter the barrel the greater the PSI, which it turn would mean the suppressor would have to deal with this increased pressure.

[alamo5000]

"In recent years, designers have become aware of limitations to suppressor structural integrity due to rapid pressure variations in the entrance chamber of their suppressors. The entrance chamber is easily visualized as a simple cylinder that acts like a pressure vessel with a hole in the far end to control the rate of pressure decrease. With gunfire, the pressure peaks almost instantaneously and drops literally in microseconds. A lot of structural stresses are applied in this short time interval. A firearm barrel can also be visualized as a pressure vessel, but one of varying length as the bullet progresses throughout its length."

http://www.sadefensejournal.com/wp/?p=1093

[alamo5000]

So in theory if you were wanting to make a 'maximum' cartridge for suppression for an SBR... you would need to select a powder based around PSI in the chamber, not always velocity. So in theory if you could come up with or choose a powder that has less pressure then you are onto something as to getting better suppression out of a shorter barrel.

At first glance Winchester 748 with a 55 grain bullet has a pressure between 39200 psi and 48500 psi... With a 55 grain bullet that will get you still in the ballpark on velocity.

IMR 4064 would be another good choice for less pressure....That is 37,100 psi to 44800 psi... you would still be right around the velocity mark as well...

That is assuming you want to load for better suppression...and not self defense or whatever.

[alamo5000]

A 55 grain soft point with Winchester 748 powder shows a MAX load of 26.3 grains and a MAX pressure of 39,000....

That is WELL below what others are at.

Varget for example which is kind of middle of the road with the same bullet has a max pressure of 49,700 almost 10,000 more PSI difference with velocity difference of only around 200 fps....

So basically you could still easily end up at the 2600 fps velocity threshold out of a 10.5" barrel with either... only the one has a lot less pressure... which should make it a lot more quiet to shoot. (with a suppressor)

[alamo5000]

In the tests above (linked below as well) their chamber pressure was measured at 55,744 psi....with m855 ammo.

http://www.dtic.mil/ndia/2010armamen...hilipDater.pdf

If you subtract 11,500 for the 10.5" exit pressure you have a difference of 44,244...divide by 10.5 and that's 4213 psi per inch of barrel that it goes down. Or in percentage terms 20% of the chamber pressure was the muzzle exit pressure.

With my theoretical ammo above using the max loads and max pressure using winchester 748 powder and a 55 gr SP bullet... the max pressure in the chamber is 39,000 psi... times that by 20% and you have an estimated 7800 psi exit pressure... which should make it about as quiet as the same suppressor on an 16" barrel using m855 ammo....and out of a 10.5 barrel you will still be up around that 2600 fps velocity...

I think it's worth an experiment...the point being trying to make my suppressor more efficient on a short barrel...and seeing if it will work like that...that's my hypothesis...

[alamo5000]