[TNC]

Anyone remember the poster who claims to have drilled his RFB gas port to the "next" size? He said it seemed to solve his cycling issues. I can't find the post at this time.

Anyway, is anyone aware of others trying something like this? Those with more RFB operational experience please chime in if you know more about potential pros and cons with this approach.

I've still not had a problem shooting two kinds of ammo that I use in my RFB...150g NATO FMJ and Federal Gold Medal 168g Sierra Matchkings. I've been surprised I haven't had to adjust gas bleed between the two.

Also, is the gas bleed design an element of a piston gun vs. DI like many AR's that use a fixed gas block or one that you can adjust? I'd never considered the difference in a piston gun to any degree and really haven't done much studying on the issue. I feel I fully understand the DI system in my AR's but hadn't really thought much about piston/op-rod guns. I'm strictly guessing here, but if a piston gun didn't have a bleed-off system to expel the excess gas, wouldn't the op-rod mechanism take a beating? And yes, I realize a DI AR can take a beating with too much gas also.

 

[ronmar]

Well most conventional piston guns typically DO take a beating. Since most I have see are NOT adjustable, they purposefuly deliver a little too much energy, expecting the excess to be absorbed by the recoil springs(and ultimately your shoulder). I don't recall seeing any system that vents, but it has been a while since I messed with any other piston guns.

The DI system on the AR does vent gas. That is what those 2 holes in the bolt carrier seen thru the ejection port with a closed bolt are there for. They release excess gas once the carrier has traveled far enough to the rear for the piston rings to expose those holes to the piston cylinder/chamber. I guess at this point it is assumed that the carrier will have stored enough energy/momentum to complete a typical cycle.

In the case of the RFB I think that extended portion on the front of the piston controlling gas inflow into the piston cylinder has as much effect on the reaction as the vent port dumping gas does.

 

[Junkie]

Can you not get enough gas into it, meaning short stroking with the regulator turned all the way in? If you can get enough gas I don't see what you'd get out of this...

 

[TNC]

I know my LWRC has a kind of "automatic" or self-regulating system where the gas not needed for cycling the op-rod is vented off out on the muzzle end of the op-rod. I didn't know if this is a common method that most piston/op-rod systems use or not. It looks like a relatively simple but effective system.

[ame]https://www.youtube.com/watch?v=3KYScJo4_s0[/ame]

On our RFB, I'm not really getting the picture of why its gas tuning system has seemed to be so problematic on many guns. On my AR's with an adjustable gas block, as long as the gas port is large enough to handle most all ammos, you have almost infinite gas control from wide open to completely shut. I've even had to drill the gas port on a couple of AR barrels because they were on the minimal edge of providing enough gas to cycle all ammos.

Since we have an adjustable gas system on our RFB's, I'm wondering if the gas port is large enough and/or the adjustment does not have enough latitude.

 

[Junkie]

My impression is that the RFB has less overtravel and is therefore more sensitive to being perfectly gassed.

I have limited experience with mine but I have plenty of gas available (the regulator is nowhere near all the way closed) so I doubt a bigger hole would help.

 

[TNC]

My impression is that the RFB has less overtravel and is therefore more sensitive to being perfectly gassed.

I have limited experience with mine but I have plenty of gas available (the regulator is nowhere near all the way closed) so I doubt a bigger hole would help.

Can you clarify your description on your regulator setting in your comment? It may be just a semantics thing, but I'm reading it that your regulator setting is toward the "closed" end of adjustment. This would allow less gas to vent, therefore more gas pressure to cycle the gun.

This is occasionally the problem in discussing the RFB gas adjuster sometimes in that it is a bleed system and not a gas pathway block adjuster like an AR. Not saying you're description was incorrect, just clarifying.

I would agree that many of the complaints/problems seem to indicate too much gas...broken recoil components and other internal components from aggressive gas/recoil along with many cases of not having the bolt fail to lock back regardless of how much gas was vented off with the adjuster...and in some cases the adjuster cap completely removed...and not just with suppressors.

Instead of the barrel gas port perhaps the gas bleed port is not large enough. Still, either the barrel gas port or the gas bleed port would seem to have been relatively simple elements for KT to diagnose along the way. Instead it seems like KT has gone through several variations of gas piston shapes/sizes to try to address the issue. It would seem simpler to have an optimally sized barrel gas port that would cycle any reasonable choice in ammo...and suppressor...but also have an equally optimally sized gas bleed port that the adjuster knob could control from fully closed to fully open.

Edit: OK I found the old thread, and it was the gas vent port...not barrel gas port...that the poster enlarged to bleed off more gas when his rifle would not ever fail to lock back regardless of how much gas he bled off with the adjuster...too much gas.

http://www.thektog.org/forum/f91/im-lost-gas-adjust-263112/

 

[Junkie]

Closed = screwed in all the way = no gas vents = more gas goes to the action.

I thought you were talking about enlarging the port from the barrel to the gas block, which would result in more gas. If you're talking about enlarging the vent port that might be helpful.

 

[issac575]

I have been following threads about the gas adjustments hear an in other forums due to the problem I have been having with my RFB. Basically I had bolt lock back with the vent cap completely removed. Based on what I learned here and in the bullpup forum I was able to understand how the gas system works in general. I did not want to go to the trouble of dremeling out the vent hole itself so I ordered a suppressor piston and got some 320 grit sandpaper. The suppressor piston was the back up in case I couldn't get the standard piston to achieve lock back at B+40 or below with minimal removal of material on the piston. Good news is the sandpaper worked great. Bad news is the suppressor piston was too large to fit in the gas cylinder. I did clean the entire gas system as thoroughly as I could before trying any of this of course. The method I used to remove material slowly and evenly was cutting a strip of 320 grit sandpaper to just wide enough to cover the gas checks on the piston and about 6 inches long. I wrapped the sandpaper around the piston so that as I turned the piston while holding the sand paper there would be light pressure of the sandpaper on the outside edges of the gas checks. I was tempted after a few minuets to go get a drill to speed my process but am glad I did not. It should be noted that I did most of the work out at my regular shooting spot in the desert. This enabled me to sand very small amounts off, re assemble the rifle and fire a round, repeating until I got failure to lock back at B+40. then I adjusted the gas per the manual until I got reliable function. I don't own a suppressor, but am surprised the suppressor piston was to large to fit. Hope anyone else that is having a similar problem can use my experience to help them have what amounts to a fix that cost only as much as you spend on sandpaper.

 

[M4V3R1CK]

issac575:
What you've done is remove the bearing surface at the weak end of the piston. This will increase the wear on your gas system significantly. There are actually a number of non-suppressor pistons available from Kel-Tec to get the gas system "range" adjusted properly.

One question, though: when you say that the suppressor piston was "too large" was it the piston lands (discs) that were too big, or was it the stem (long skinny bit)? either way, your sandpaper approach would probably work to make it fit, despite my warning against using it to make tuning adjustments.

 

[TNC]

issac575:
What you've done is remove the bearing surface at the weak end of the piston. This will increase the wear on your gas system significantly. There are actually a number of non-suppressor pistons available from Kel-Tec to get the gas system "range" adjusted properly.

One question, though: when you say that the suppressor piston was "too large" was it the piston lands (discs) that were too big, or was it the stem (long skinny bit)? either way, your sandpaper approach would probably work to make it fit, despite my warning against using it to make tuning adjustments.

Refresh my memory...were you the poster working on some kind of auto-gas setup for the RFB? If so, would it be something like the one I mention above on the LWRC?

 

[issac575]

M4V3R1CK, Thanks for the info, see the posted photo for where I was careful to only have sandpaper contact the piston. (yellow lines) Also it is worth noting that I removed very little material, really more of a polish than anything. I have removed more material when sharpening pocket knives to be honest. Please let me know if you think this will cause problems. Thanks.

I am not the one working on auto-gas, sorry.

 

[M4V3R1CK]

Ah! My apologies, issac575! You're good! When Supervisor42 was working on his rendition of the pistons (which I currently have), he found that a .0005" variance in the area you sanded made a HUGE difference in how the weapon reacts to the gas setting. I thought you were sanding on the other end, which could cause some problems. Where you sanded is fine, although it does cause more gas to vent towards the shooter. Glad you found a solution to your problem.

TNC: That would be me. Sorry I've not made much progress. I've been having some issues in the rest of my life causing the gas system to get temporarily shelved. It WILL get done though, and no, it will be nothing like anything you've seen before. I promise. There are a small handful of people on here that have seen my initial designs. Those might have worked, but were entirely too complicated to make. My next step is to simplify the mechanism and find a machinist that isn't too busy MOVING HIS HOUSE *cough* CyberScoper *coughcough*

 

[issac575]

Yeah, we all have life get in the way sometimes. Luckily my gas problem was easy to fix once I understood the mechanics of how it operates. The extra gas isn't a big deal to me, the difference is minimal and my everyday glasses are balistic rated. Hope you get your project completed though.