M*CARBO Brotherhood

A Basic Understanding Of Blowback Weapons

As I read the various threads regarding recoil, cleaning, dirty brass, and after market parts I realize that not everyone understands the mechanical issues of a blowback design. (If you do then skip this thread)

First, a blowback weapon utilizes the spent cartridge case as a piston to cycle the action. This is why most blowback weapons do not have a locking bolt and the brass is so dirty.

To keep the weapon from “running away” and venting burning propellant outside of the chamber the cyclic rate of the bolt must be slowed down. There are two main methods to this: friction as the Thompson Machine gun uses and inertia as found in your Sub2000.

However, this results in two very different “types” of recoil: gas and mechanical.

Your muzzle brake redirects the forces of the gas at the end of the barrel up and back, thus pushing the weapon down and forward…this method will counter the effects of gas recoil by providing a counter force. This will do nothing in regards to mechanical recoil!

That heavy counter weight moving back and forth in your Sub2000 generates some significant mechanical force. You can slow it down by adding more weight, or you can cushion the impact by adding a buffer pad.

Keep in mind that by increasing the mass you are reducing the cyclic rate and trading a fast “whack, whack, whack” for a heavier, but slower “thump, thump.” (Sort of like the difference between shooting a 9mm vs a .45acp). This is exactly why KT makes a heavy counter-weight optional part and MCARBO makes the heavy charging handle.

You are also increasing the load on the return spring. This will result in the spring over time weakening faster and you will eventually see your mechanical recoil increase as resistance is decreased!

Another issue is that while the Sub2000 relies upon inertia, there is some friction between the mass of the moving parts. Excessive polishing of the receiver tube and bolt group (or just time and normal wear) will also increase your felt mechanical recoil.

I know this is basic mechanical physics for many, but I hope it might help some of my brothers whom might not have the same background.


I thought I would post this explanation as to why your S2K is so dirty and why you cannot get the same recoil reduction as you can from other types of firearms!

Good news is blowbacks are stone axe simple, reliable, and function well (even when dirty). Just take care to make sure the build-up of carbon on the bolt face and extractor claw get scraped away and you can shoot 1,000s of rounds error free!

It is also the reason why these weapons should not be “over lubed” it is a dry firing weapon and over lubrication can reduce friction and create a build up of gunk that can impede function.

Hope this info helps someone?


@Johnksg awesome write-up sir. I don’t over-lube any gun I own, in fact I’m probably guilty of under-lubing but I do clean the gunk off. Great blow-back explanation as well!


@Kona wet lubrication is the enemy of Thomsons, Stens, Sub2000s, (and grease guns) @godallmighty they just need a good scraping every so often.

They are in many ways an ugly, simple system, that I find somehow beautiful in their simplicity and mechanical engineering!


Excellent write up, well said. Thank you.


@Patriot1 Thank you!

It is basically physics…Newton’s 3rd law, and why shooters see a greater difference in @ChrisNelson MCARBO muzzle brake and not as much from the heavy charging handle. M*V=F(N) or in other words you are slowing down the velocity (v) but at the same time increasing the mass (m) which results in felt recoil or force (F).

What you do realize is the difference between a “slow, heavy weight” vs the difference of a “faster lighter weight” in a reciprocating motion.


Great stuff. Thanks for the info. I have all the MCARBO stuff including the recoil buffer, the heavy-weight charging handle, and the muzzle brake on my Sub2000.


Outstanding ! Information as Always from the Great Master! Now I have go to the lube Topic’s for the best to Use in this Weapon, Thanks Again.


@DivaMarie every so often these types of weapons need to be completely torn down and cleaned in a sonic tank and scrubbed.

In between I just scrape the bolt face and under the extractor and clean the bore.

A dry lube works best, but the tolerances are a bit loose and some friction can be good!

I just wish the trigger group was in a sealed unit that could be detached…sigh


@Johnksg, I agree with you 100%. After my mishap ‘trigger job,’ I am scared to touch them. LOL



I installed the Ruger BX-trigger upgrade on my Ruger 10/22 Takedown. It was so nice to basically just buy it off the shelf and drop it in. :grinning:

I don’t know why they don’t put it in there in the first place, but it was so easy to upgrade!


Have you checked out tandemkross.com ? They have some neat products.


Are the Dry Moly’s Acceptable, or Hornady One Shot Cleaner and Lube for Bolt and Tube?
After the Solvent, CarbClean,Brake Clean and Scrubbing? This is a hard egg for me to get behind, Having been a Mechanic for 50+ ! Oil is Good, Lack of, and Dirty Oil and Heat and Excessive Wear and Premature Failure.:smile:
Thanks for the Heads up and help with the Above’


I WAS a mechanic (in my younger days) and I agree with your logic. It seems that with gunsmithing there are ‘different basic laws.’ Bob Dunlap from AGI will give one his theory. In my opinion, if one alters one part on a gun another area is affected. All guns wear. Some better than others. Consult highly acclaimed gunsmiths and do your homework. Hope that can help a little.


@DivaMarie @lonewolf honestly I think any modern cleaner that did not use harsh solvents that would hurt the plastic would be acceptable.

I use hoppes foaming bore cleaner and rem-oil. The main thing is to wipe away all excess lubricants (difficult to do on the S2K) when you are finished it should look “dry” not “wet”.

The two main problem areas on and blowback weapon to be addressed is the bolt face (especially under the extractor) and the feed ramp.

Like many of you it is hard for me to accept a dirty weapon. However what I know of military weapons of this type is they are generally very reliable and not very finicky without a ton of maintenance.

Some of the issues I have read on other threads sound like cyclic rate problems…either it has been sped up due to excessive polishing an lubrication or slowed to far down by excessive counter weights that result in prematurely weakened return springs.


One issue with the heavy recoilless charging handle is the weight is perpendicular to the bolt and counter weight. This can cause greater wear on the leading edges of the counter weight.

The top bolt assembly is actually from an older weapon with standard charging handle. On the other you can see a significant amount of metal has been removed due to what I believe the heavier charging handle.

In effect it “self-corrected” and any reduced recoil from a slower cyclic rate is negated by the decreased friction from the wear!


@Johnksg I went about 2000 rounds without cleaning my sub-2000 and it ran just fine. It really is a dirty gun that will just continue to run without detail cleaning every range trip.


@Johnksg I plan to go back to 2FCH. I don’t hate the RCH but it’s just too big and I don’t see it’s benefit.


@Kona my only concern with 2FCH is I wouldn’t be surprised that with heavy or prolonged use over time we start hearing about charging handles snapping or breaking.

Essentially it is a longer “structure” under a lot of reciprocating force and a higher CG. Longer structures are inherently weaker.


@Johnksg I’ve broken 2 of them but I got my money’s worth out of them. I’ll just buy a pair “2 is 1, 1 is none” and all that jazz.