[Component Selection and Compensators (Long post)]

With the question of “Which is the best powder to run in an open gun” that keeps coming up, I thought I would try and help out with a summary of the basics on why certain powder and bullet weight combinations should work better with compensators. There really isn’t anything new here that hasn’t be touched on before, I just tried to list the key points and add in some science behind the why.

Sorry this is a bit on the long side and probably has more information than you probably want to read through, I tried to keep the math to a minimum.

What We Know

From years of people trying different combinations of powders and bullets it has been found in general:

For compensated guns:

Run low weight bullets at high velocities.

Use slower burning powders.

Increasing the Power Factor makes the compensator work better.

For non-compensated guns:

Run heavy bullets at lower velocities.

Use faster burning powders.

Run as low of a Power Factor you can get away with.

So why is this true? Here is the breakdown on why different combinations tend to work better with compensators.

Recoil Basics (The boring stuff)

Recoil or momentum = Mass x Velocity.

The total momentum (recoil) = (Mass Bullet x Bullet Velocity) + (Mass of Propellant Gas x Gas Velocity)

Note: Mass of Propellant Gas = Mass of powder charge when 100% combusted.

For a given Power Factor the first part of the recoil equation we can’t do anything about (Mass Bullet x Bullet Velocity) this will be fixed. The second part (Mass of Propellant Gas x Gas Velocity) is where the compensator comes into play and is the part you want to use or lose. The momentum of the gas will add to the recoil from the bullet, much like a small rocket engine going off in the barrel.

Velocity (Still boring…)

As some people have probably noted that doubling your powder charge will not double the bullets velocity. All things being equal (caliber, bullet weight, barrel length…) to double the bullets velocity you need to quadruple the averaged pressure in the barrel. The faster you try to push the bullet the more “velocity inefficient” the load becomes.

(Kinetic) Energy = ½ x Mass x Velocity^2

The energy put into the bullet is equal to the average pressure acting on it to push it out the barrel. So from the energy equation you can see when you double the bullets velocity you have four times the energy or average pressure.

As an example the 22-250 round is probably one of the most inefficient rounds out there by ratio of (bullet weight x velocity) to powder used.

Hang in there this is going someplace I promise….

Bullet Weight (Getting better…)

This unbalanced bullet velocity to pressure relationship is the reason why lower mass bullets work better with compensators. To maintain a given Power Factor when you decrease the bullet’s mass you need to increase its velocity, when you increase the velocity you disproportionately have more gas and pressure.

If you cut the bullet’s weight in half you will need to double the velocity to keep the same Power Factor. When you do this because of the (Velocity^2) relationship you will have twice the gas energy to work the compensator.

Pressure

You often hear some say “You need to up the pressure to get the compensator working”, which is true because higher pressures will produce a higher mass flow rate of gas out the barrel and more energy.

People often look at peak pressure numbers from reloading data to make their powder selection thinking higher peak pressures will make the compensator work better, which is actually the complete opposite direction to go with, you want the powders with lower peak pressures. Now intuitively this may sounds backwards from what you would think….Here’s why.

It’s the muzzle pressure in the barrel just as the bullet leaves that you want to increase not peak pressure. Fast powders with high peak pressures burn out too quickly and will have lower muzzle pressures than slower burning powders.

Higher muzzle pressure = Higher mass flow rate of gas = Higher gas energy to work the compensator.

The Curve (The good stuff)

To illustrate the differences in powder burn rates here are two pressure curves generated from Quickload software showings Unique (fast) and AA#7 (slow) with the same bullet weight and muzzle velocity.

The area under the pressure curve should be about the same for both graphs. You can also see that the slower AA#7 has a muzzle pressure 1211psi (22%) higher but with a 3855psi (10%) lower peak pressure than faster burning Unique.

Here is one more example taking it to the opposite ends in load combinations, a light bullet (115gr) using slow powder (AA#7) and the other a heavy bullet (147gr) with fast powder (Unique).

As you can see the light bullet and slow powder combination produced almost a 3100 psi higher muzzle pressure or 73% more gas energy than the heavy-fast combination even though their Power Factors are the same.

Powder

So how do you select which is the best powder to use with a compensator? The goal is to get the highest mass flow rate of gas, which means using the powder that will yield the highest muzzle pressure (not peak pressure).

If the powder is too fast:

Lower muzzle pressures.

May over-pressure before getting to desired Power Factor.

If the powder is too slow:

Lower muzzle pressure.

Gun runs dirty from unburned powder.

May run out of case volume before getting to desired Power Factor.

When running the slower powders and how completely they will burn can dependent on many factors such as bullet seating depth, bullet diameter (.355 vs .356) bullet weight, free-bore length, primer, crimp…. One person may be running brand XX powder without any problems but with another person unburned powder may start to cause malfunctions.

If your current 9mm/38 super load is showing high pressure signs in small rifle primers you may want to think about switching powders there really isn’t any reason for it unless you’re trying to reload on the cheap. In most cases you should be able to use a slower powder to reduce the peak pressure and still achieve the same or greater muzzle pressures. Unless you’re running 185 PF in a 9mm then you’re just lucky your pistol is still in one piece.

The Real World

Unfortunately even though you can fairly accurately predict which powder and bullet combination will produce the highest muzzle pressure it does not mean that’s the one that will ”work” the best for you. The dynamics of the gun cycling will change from person to person, some people will put a death grip on the gun others hold loosely. Then you still need to get into a whole other topic of tuning with the springs.

The compensator design also comes into play, higher muzzle pressures may come across as feeling too harsh and lower pressures may just feel better. Then finally if the compensator isn’t very effective to begin with then running more gas may just make things worse.

So at the end of the day it’s still going to be trial and error with different combinations and run whatever works for you, there isn’t going to be a magic bullet/powder combination that will work the best for everyone.

Going with the crowd

For 9mm major and 38 super like most everyone else I would stick with 115-125gr bullets and start off with the slower powders.

Looking at load data that other people have listed will help narrow it down. If you see something listed like: 115 jhp - 9.5gr of XX- 170PF-“burns clean” would probably be a good indicator of a load to start paying around with.

By the Numbers

Here are the results of some powders generated from QuickLoad for a 5” 38 SuperTJ running 124gr (.356) Zero JHP at 170PF.

Sorry I didn’t have any data available on AutoComp or HS-7, I also tossed out some loads with over 115% filling ratio (compressed load)

WARNING: DATA LISTED HERE IS FOR COMPARISON ONLY – DO NOT USE!! Refer to current handloading manuals.

Looking at the table and going with the criteria of having the greatest muzzle pressure we can pick out some powders that in theory should work well with compensators. Sorry if your favorite powder didn’t make the list, if it works for you go with it.

1)Rottweil P806 6942psi

2)Vihtavuori N105 6719psi

3)Alliant 2400 6676psi

4) Accurate No.7 6603psi

5) Lovex D037.1 6603psi

6) Alliant POWER PISTOL 6522psi

7) Accurate No.9 6438psi

8) Lovex D037.2 6438psi

9) Somchem S221 6349psi

10) Vihtavuori 3N38 6303psi

[STI trubore barrel]

Jediwarrior,

I understand your concern but I just go about it a different way to get the same results.

I like to fit the frame and barrel to have the low lug impact the Vertical Impact Surface instead of the horizontal surface (Schuemann barrel fitting method). It will stop the barrel in the same place in link down as if it hit the horizontal surface but doesn't put as much stress on the link and slide stop pin. The barrel will not contact the frame at 8 and 4 o'clock which can be verified as I don't have any marks on my barrels as shown in your picture.

As I had mentioned I do sometimes need to remove material on the top of the frame (4 and 8 o'clock contact area) for the barrel to link down enough and get 0.010 in. clearance with the upper lugs. It just turns out with some combinations of barrel, frame and slide fit there isn't enough clearance otherwise.

As you said its an art, no to guns are the same and if its running ok go with it.

[STI trubore barrel]

Hi Tommy,

A bit off topic, but what was the depth of the Horizontal Impact Surface (HIS) on your new build? This is the distance from the top of the frame to the bottom of the feed ramp. This distance for STI frames varies from 0.295 in. to 0.305 in.

There should be a minimum of 0.010 in. clearance from the top of the barrel and the inside top of the slide.

If cut too deep, you could have cycling problems in the future as the barrel drags on the inside of the slide at the 8 and 4 o'clock positions.

Schuemann's instructions of a depth of 0.315 in. is wrong.

For my STI 38super with the Trubor full ramped barrel the distance from the top of the frame to the bottom of the feed ramp cut came out to .321in. with 0.010in clearance at the top with slide stop pin in. That leaves also about 0.010in. clearance under the ramp at full link down.

I have about two thousand rounds through the gun so far without any problems. I don't thinks the depth of the cut its all that critical as long as the barrel is impacting the vertical surface. If there is 0.0005in. or 0.050in. clearance under the ramp I don't see how operationally the gun would know the difference other than reducing the vertical impact surface area a little, but you need to radius the top edge anyways.

I also found that with STI frame and Trubor barrel combo you need to do a little clearance cutting at the top of the frame to get the barrel to link down enough.

Everyone has their own way of doing things, I just like to put a little extra space under the ramp to leave a clearance for gunk buildup.

Sorry for the highjack back on topic:

What Jediwarrior said, just get the Clymer reamer you most likely will not need to lengthen the free bore unless you want to play around with reducing peak pressures or something.

[.38 Super Comp]

38 supercomp and TJ brass will fit in the 223 shell plate, regular 38 super brass won't.

[Ion Bond vs CERAKOTE]

Cerakote seems to hold up well on the comp ports but if it touches any part of a holster it will show wear signs fairly quickly. I think builders like cerakote because the prep is quick with just sand blasting unlike poslished hard chrome where every little scratch will show through.

[A somewhat in depth look at how to tune an AR]

I veiw the comp like other parts of the AR, it needs to be cleaned. Just about any rifle comp will build up carbon if you shoot enough. Just hose it down with some oven cleaner let it sit a bit and scrub with a small brush (old bore brushes work geat) then rinse. May need to use the edge of a brass rod to scape the hard to reach spots but it not a big deal, you really only need to do it once or twice a year.

[.38 super load suggestion]

For 38 super with 5" barrel of the VV powders:

N105 - Will produce the lowest peak and highest muzzle pressures, works great with a comp but you will fill the case up.

3N38 - Would be my next choice still fills the case up a bit.

3N37 - I would put in the middle

N350 - About the same as 3N37 just gives a bit higher peak pressure.

N340 - A bit too fast for 38 super, high peak pressures.