Important information on the SF configuration

[TerraShrewsbury]

as per Alex:


We have not put the SF model of the XCR on our website yet because it was not fully tested. With the 14.7" barrel with the permanently attached flash hider, we're finding that the gas port is too close to the chamber and the end of the barrel is too far from the gas port. The result is too much chamber pressure while the case is being extracted. W're ripping the rim off some cartridges. Not good. We've tried quite a few things to solve the problem but it may not work right.

We have a couple options which work great. We can use a standard length gas system on an mini upper. This leaves the gas tube hanging out a couple inches. We can make special gas tubes for this.

We can also sell 14.7" barrels with permanently attached muzzle brakes for the the standard upper. These balance quite nicely. We'll try a few more things to see if we can get the SF to work. I'm sure we could get the SF to work with a shorter barrel. We'll keep trying.

[kehvN]

*sends positive problem-solving vibes to RobArm* :crossedfingers:

[LurpyGeek]

Appreciate the update.

How much would a 14.7" heavy barrel in 5.56 for the standard upper with a pinned (user supplied) muzzle brake cost?

[DSM]

Might try one of the XM177 style flash hiders then. Shorter barrel, longer flash hider.

[Underground]

as per Alex:

[b]
We have not put the SF model of the XCR on our website yet because it was not fully tested. With the 14.7" barrel with the permanently attached flash hider, we're finding that the gas port is too close to the chamber and the end of the barrel is too far from the gas port. The result is too much chamber pressure while the case is being extracted. W're ripping the rim off some cartridges. Not good. We've tried quite a few things to solve the problem but it may not work right.

We have a couple options which work great. We can use a standard length gas system on an mini upper. This leaves the gas tube hanging out a couple inches. We can make special gas tubes for this.

We can also sell 14.7" barrels with permanently attached muzzle brakes for the the standard upper. These balance quite nicely. We'll try a few more things to see if we can get the SF to work. I'm sure we could get the SF to work with a shorter barrel. We'll keep trying.

Interesting problem. Sounds like you would need to vent some extra gas at the gas block maybe, with some going to the piston and some outside.

[kehvN]

Ummm... maybe a larger overall gas block opening for all settings?

In my very limited understanding that would increase port pressure and reduce chamber pressure. Am I right on this you experts on here???

To compensate for the higher port pressure you could then offer a stronger buffer spring for the 14.5" barrel config. Just thinking out loud here...

Of course you guys are the experts who made this awesome platform and I know you'll get it worked out. We're anxiously and patiently waiting, heh.

It makes me happy to know that the case is what's being affected and the chamber/upper is holding up well!!!

[krow43o]

I think by what Terra is describing, the position of the gas port in relation to the chamber and muzzle is creating a timing problem for extraction. When the bolt is unlocking and trying to extract the casing, the pressure in the chamber is still too high. Effectively locking the brass casing sides to the chambers walls.

[kehvN]

I think by what Terra is describing, the position of the gas port in relation to the chamber and muzzle is creating a timing problem for extraction. When the bolt is unlocking and trying to extract the casing, the pressure in the chamber is still too high. Effectively locking the brass casing sides to the chambers walls.

Ok... so if that's the case... if the action is ahead of timing it means the port pressure is too high. Too high of port pressure can cause the action to begin prematurely, causing the extractor to engage while the chamber pressure is too high. This happens on poorly configured AR varmint rigs where the barrels are longer than 20" with no modifications to compensate for the higher port pressure. Extending the gas tube OUT an inch usually cures that from what I've read. I guess really it could go either way, eh?

1) The port pressure is ok but the chamber pressure is too high because of the longer barrel. Or...
2) The port pressure is too high so when action begins too early, by consequence, the chamber pressure is too high.

Either way adjusting the gas valve would solve the problem right?
1) Increasing the port pressure solves this. It allows more gas to flow through the port which conversely lowers the chamber pressure. Just from my abstract thinking if the currently configured weapon functions "better" on the "higher" (larger diameter) gas settings... this could be the solution.
2) Decreasing the port pressure solves this scenario. It'll "slow" the action. So if the weapon functions "better" in its current configuration on the "lower" (smaller diamater) gas settings... you know the drill.

My money is on scenario 1. Just thinking about it, longer barrel, gas tube further back than in the standard configuration. Yes the action may be happening too soon because of how much closer the gas block is but adding more buffer tension and/or allowing more gas in would, hopefully, solve it and allow the short gas block configuration.

I don't want anyone reading these replies to think we're trying to tell anyone how to do anything... It's just to create a dialogue for all of us (me mostly, heh) so that we can learn more about how the weapons operate. I know that I definitely need it. Also it keeps the thread bumped up to the top. *shrugs*

And if I'm wrong on any of this please please please someone with some knowledge chime in!

[piusmilitis]

Excuse me if this is a dumb question, but whats the big deal/ so special about this "SF" variant of XCR.

I'm son confused with all the XCR's now, just how many uppers are there now and what can/ can not go with what???

There's still just one lower right?

Chamber pressure to high, no good. Does Robarms use fluid modeling software to model the gas block/barrel/chamber pressures? I kinda wondered if there was just some standards in the industry or if people model that stuff out when doing this sort of design. If so I'd be interested in knowing what package you guys use...

If you guys need more Engineering Muscle I'll have my MS in Mech. E by 2012! Just say'n . . . You know . . Just FYI . . . ;D

[Underground]

Nah, the end of the world wil be here by then.

I think the whole point was to have a shorter gas system and handguard with a non-NFA length barrel to minimize weight. So you don't really want to make the gas system any longer.

If it's ripping rims off, you obviously don't need any more gas to the piston.

So you've got extra gas that needs to go somewhere right before the action starts cycling. I don't see much option other than bleeding it off at the gas block without changing the basic configuration. Of course you'd probably have to baffle it a little somehow too. I'm assuming that could do some damage to your hand coming out of a gas port sized hole.

It's an interesting puzzle.