Physics of a compensator

[eric nielsen]

There are a lot of good points on this thread already, so I'll just cherry-pick a few to agree with. I've been shooting open guns since 1991 with various calibers & comps & ports if that's any indication.

In general I'd say that now, with power factors around 172 needed for a match, a 115 grain bullet with more than 9 grains of powder, OAL 1.230" or so, is my favorite load combination. My gun configuration now [with no big desire to try other combos out there, & there are many] is a 5" slide lightened to under 11oz, a standard twist standard size 5.5" barrel, a steel cone comp with 5 chambers, and no porting. Sideports on the last 2 chambers. Also nice is 2-4 small TJ-style holes thru the barrel & slide although I don't have that now. I'm so happy with the barrel/cone-comp workmanship on my gun I don't want to mess with it.

My humble opinion the most important thing the comp does that actually makes the gun EASIER to shoot [as opposed to not flipping but maybe being a bear to shoot] is that the gas slams into the baffles of the comp chambers and pushes the gun forward. I think it takes medium-to-large size ports to do this, and at least 4 chambers worth, assuming you load more than 7 or 8 grains of powder, which most of do. I think that side-ports help the efficiency of this process by venting more of the gas before it can follow the bullet out the end. Possibly the side-port does this better than opening the top ports because of some sort of flow dynamics down in the low parts of the comp baffles - as in - adding real or virtual surface-area to the baffles. All else being equal, adding side-ports to your comp will definitely take some kick out of your hand, will not add/subtract noticeably from muzzle flip, and will not subtract much or any from your bullet's muzzle velocity.

Because of the mechanics of how we hold the gun - barrel above wrists, wrists above elbows, wrists & elbows below the shoulders - I think it's almost impossible to prove that one comp does one particular thing BETTER in terms of physics. The best you can ever say is "this comp works better than that comp at the same power factor" - although that's rarely agreed on. Best you can realistically say is "I like this comp better, it makes it easier for me to shoot the next shot". Having said that I think there are comp designs that have proven themselves very well. You can look at comps by Brazos, Dawson, EGW, Bedell - all of those comps are doing at least 2 things very well: reduce felt recoil & reduce flip.

I think that it is possible to make the comp work TOO well, although this is rarely tried [for long] on a semiauto pistol. It almost requires making the comp wider than the slide and taller than the top & bottom of most normal comps. At some point a comp is moving so much gas into pressing the gun forward that there is not enough energy to work the slide back & cycle the gun. This is best seen by looking at Open Glocks, where you can quickly run past the limits of what is too much moving mass [heavy comp] and what is too much forward gas pressure [needing a spring so light that the slide backs out when you pull the trigger]. Tougher to get that carried away on a 1911/2011 but it's been done.

There are other load combinations that work for some people. 124gr bullets and less powder is one. I tried and really liked a fast powder which is IMR 7625. It took less powder, kicked back a lot less, but flipped more. Less "rocket effect" and less compensator effect - both. It also beat the heck out of the rear end of my 38super cases so I stopped using it. I like brass to last more than 3 firings. Another load combination is a 124 passing under a LOT of barrel-porting and thus still needing around 9 grains or more to make PF. I don't like these if it involves true nozzled Hybrid ports because of the very high amount of noise and the incredible shock wave. After the classes I've taken so far, I'm even less inclined to let waves of concussion go slamming into me, especially the ears and eyes which you should bear in mind are PARTIALLY protected by shooting glasses, plugs, and muffs.

Another combination you can shoot is a shorter-cartridge setup using 9x19 cases. This narrows your powder choices down and [my humble opinion] makes it more important that you vent less gas before the expansion-chamber comp, and more gas IN that comp. Also the average 9Major gun [i'm sure there are exceptions] will dance & jump in your hand more than the exact same design with Super/Supercomp ammo, so you may want to consider going with the more established "stable" and "soft" setups, as in, 5" slide, 4+ chambers, steel comp, etc.

One thing that was mentioned was the slide hitting full back & what that does to recovery and shooting fast. More than just the comp and the length/weight of the slide is involved with this. There's also the roundness of the FP stop, the strength of the hammer spring, the strength of the slide spring. Maybe in that order [insert argument here].

One thing that most modern comps and loads do is give you a gun that operates well when the slide is down under 11 oz, and apparently too erratic & prone to primer swiping/premature unlocking if you go much below that. And [my observation] more flip & felt recoil if the slide is much heavier than 12oz. My STI/SA-Custom and my previous Caspian open guns both had 10.5 oz slides & I liked them better than every gun I compared them to. I use a 9 or 10 lb slide spring for both. A 1911/2011 that requires an even lighter spring than this is going to be hit-or-miss about closing the slide with a new round in the chamber.

EDIT to say - I'm not sure how much we need to lighten slides because of INCREASED back-pressure on the case caused by compensaters, or whether this changes how much you should lighten the slide of a heavily-ported gun vs a many-chambered gun. I'd say that I know but now there are so many light-slide Limited guns now I just wait for a gunsmith to answer.

Edited May 6, 2009 by eric nielsen

[shred]

I used to worry about the physics of comps and all. There's pages of discussions back in the archives, but now I don't think it matters that much.. pick one you like that works ok with a load you like and the rest of it won't make a dime's worth of difference. Max kicks butt with pretty much a standard TruBor comp. Todd shoots a modified Dawson. BJ, the Brazos. JJ shoots the Limcat. Eric shoots the Tanfo with a ton of little holes. Chris the SV with a bunch of holes.. and so on.

The 9x25's had the best comp action ever, but it still wasn't worth two rounds in the magazine.

[Davidp1911]

I used to worry about the physics of comps and all. There's pages of discussions back in the archives, but now I don't think it matters that much.. pick one you like that works ok with a load you like and the rest of it won't make a dime's worth of difference. Max kicks butt with pretty much a standard TruBor comp. Todd shoots a modified Dawson. BJ, the Brazos. JJ shoots the Limcat. Eric shoots the Tanfo with a ton of little holes. Chris the SV with a bunch of holes.. and so on.

The 9x25's had the best comp action ever, but it still wasn't worth two rounds in the magazine.

I agree, the possibilities are endless regarding specific comps/bullets/powders combos. I've always looked to the top shooters for insight on what to use and go from there - unless you're into experimenting as a hobby. We as IPSC shooters are always looking for that "edge" in technology to help us be competitive but I believe a large part of the game is physical ability and adapting to your gear.

Edited May 7, 2009 by Davidp1911

[davecutts]

Once again it comes down to the indian.

Jim makes a good point about the loud guns "seeming" flat to the shooter, who is blinking during recoil therefor deluding himself into thinking the dot isn't leaving the scope.

[HSMITH]

I've used a good selection of comps. As far as 'the best' I like the Bedell and Brazos comps about equally, both are fantastic. My second choice would be the Brazos version tru-bor comps. My third choice would be a long STI factory tru-bor comp with a few personal mods. Like most other things I have spent the cost of a nice used car trying to find the 'best'.

We can argue 'indian not arrow' and 'gear queer' all you want, but until you shoot the EXACT same gun with the EXACT same load and only a comp change you might as well be whizzing in a fan. Your comments are useless. If you can't call a shot your comments are useless. Not trying to be a Richard Cranium, but a lot of people comment on things that they don't have pertinent experience with.

My personal Open gun runs a Bedell Ti comp, no ports, bull barrel and a total top end weight of exactly 21 ounces. Food for thought...

[GentlemanJim]

That kinda sums it up.

As of now no real testing that can produce exact results with no variables has been done.

Until some person uses a test pistol, in a special rest with multiple accellerometers down loading to a PC or such

And creates real and repeatable solid data...it will be nothing but he said she said or it FEELS like

Most comps on the market are designed around only TWO things...(notice I said most)

1 what does it look like

2 what is the cheapest to machine

Just the cold hard facts

Jim

[davecutts]

We are all remise the question was about the physics, not what's better. So back to theories on how a compensator works.

[benny hill]

Hey little kahuna, take a look at the open gun cycle on my web sight. You will notice it has NO movement untill the slide hits the frame. It is a 115gr bullet over 12 gr. acc. #7, a very slow powder. The big rage is to use a really light recoil spring in a open gun but the lighter it is , the more lift it has when the slide hits the frame harder. Put in a spring that just lets the slide kiss the frame & have less lift.

[Shay1911]

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

[ExtremeShot]

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

Very cool picture. Thanks for sharing.

[davecutts]

while it is an interesting photograph, recoil hasn't started yet so of course it's flat.

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

[Shay1911]

while it is an interesting photograph, recoil hasn't started yet so of course it's flat.

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

Another one:)

[yoshidaex]

while it is an interesting photograph, recoil hasn't started yet so of course it's flat.

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

Another one:)

Is that set screw backing out?

[joecichlid]

yoshidaex, looks to be the windage locking screw on the C-More is backed out a few full turns. My guess is someone was adjusting their dot maybe? Even at that, the screw is backed out way more than is needed for that.

Joe W.

[wombat]

while it is an interesting photograph, recoil hasn't started yet so of course it's flat.

Cool pic of an open gun firing. Notice the bullet is out of the barrel and the gun is flat.

What do you think causes recoil?

[notasccrmom]

What do you think causes recoil?

The slide hitting the frame at the end of it's rearward travel. If you watch a slow-motion video, it becomes really clear.

[davecutts]

the reciprocating slide is one of the biggies.

[Harmon]

the bullet leaving the barrel at a high rate of speed causes the recoil

auto pistols spread the recoil out over a longer time frame than revolvers is what causes the recoil for equal powered cartridges to appear to be less severe in auto pistols.

now, the frame being impacted by the 10 oz slide traveling pretty fast is a cause of some muzzle flip...not recoil.

[shred]

the bullet leaving the barrel at a high rate of speed causes the recoil

Technically, it would be the conversion of the stored energy in the powder and the resultant acceleration of the various bits and pieces.

Flip in a semi appears largely when the slide hits the frame. Again with the changing of force vectors and accelerations of various bits and pieces.

[Jody Waring]

Benny, in your guns have you found a "sweet spot" for just the right spring rate to use?

[ong45]

So to get an open gun to shoot "flat" you can :

1. Tweak the load to get more gas to work the comp, thus bleeding off slide velocity to reduce the energy that gets transferred to the frame.

2. Use a heavier comp to get the same effect.

3. Lighten the slide to reduce the mass slamming into the frame.

But you also want to reduce the dip you get when the gun slams back into battery, so you run a lighter recoil spring and lighten the slide.

Which might make stripping the next round out of the magazine iffy , especially the first round of a 30 round big stick.

If your magazine spring is kinda weak then the fast light slide might outrun the mag before it has a chance to get the next round up, oh yeah did you use a shorter mag spring so that it feeds easier with the light slide/recoil spring combo?

Got a headache yet ?

[Braxton1]

Speaking of 9X25, I built one of the first 357Sig comp guns back in the mid-90s. I had Irv Stone III take one of his 9mm barrels and rechamber it to 357. I put a comp on it that a friend built. He had never built a comp before, so he made some "mistakes". From the outside, it looks like a conventional dual-port comp, but when you look on the inside, it's just one big expansion chamber with two ports. I was thinking that this wasn't gonna work at all.

Turns out, with 95s & 115s loaded to Major, the gun actually recoiled down. It would noticeably dip. I had to go to a 124 to get it to track right.

...And that was with a messed-up, badly designed comp. So I would agree with the others who said that the expansion chamber design is at least as-important as actually port design/placement/numbers.

[benny hill]

In my open guns w/a 11oz slide , I like a 10-11 # spring & a 9# w/a 10 oz. slide

[pmd]

Is that set screw backing out?

Actually that C-more screw did start to back out and has a few times since - I need to add some Loctite or fingernail polish to it.

[newshooter]

I shoot a 5" open with a Shuemann 4-port barrel and Bedell 7-port steel comp shooting a 115 gr. Montana JHP using AutoComp (168pf) - this is by far the best combo I've ever tried to date. It shoots amazingly flat and soft! I've tried other powders over the years like N350, N105, 7625 and LongShot but so far the AutoComp is working out great.

Is that for 38 super? or 9 major?