Long stroke SureFire Carrier

[Diamondback]

Would anyone like to look deeper into some things?

I'm game to see where the rabbit hole goes, but you'll have to put it in Village Idiot terms if you want me to keep up since I'm rather new to the platform, with only one build "cosmetically complete" (needs lubing and staking, which I'm going to do around a week or two before first-firing trials) and a second (the aforementioned 10.3 pistol) in test-fitting and weight-and-balance while I wait for my pro gunsmith to get back to me about some "Can't Be Done In A Studio Apartment" work on the upper.

[tom12.7]

Without the intent of jumping forward too much. There are some things that we can look at for the base system. We already knew when peak bore pressure occurs at a time for the combination for most any given barrel length. If the action was cycled manually at a different time than if done semi automatically or at a time before or later than that, it changes some things when looking at stresses and strains imposed by those for the unlocking sequence and extraction/ ejection. The Barnes 110 with H110 powder carbine gas A5H2 16" becomes more similar to the 20" RLGS and A5H2 with some of the .222 loading, not .223 loads. Combinations similar to this offer low stresses in the system compared to some others.
Magazines may not all offer the same round presentation, in time.
There are some ways to improve this.

[MistWolf]

The overall gas drive (total area under the PV curve) can be made right by adjusting the gas port size, but that is not the same thing as optimal timing.

Gas port location is the most important factor controlling the timing.

My thinking is this- If carrier speeds are the same, timing is about the same. I'm also thinking that since the gas port is tapping into the barrel where the pressure is much higher which requires a smaller gas port to control flow, the flow hitting the expansion chamber hits the carrier with a harder pulse. Sort of like moving a ball with a slap instead of a push. This shorter, harder pulse would increase stress on the bolt lugs.

Does this make sense, or am I off my rocker?

Agreed, but that doesn't mean a 16" carbine is the optimal configuration.

I agree. The span of function is much narrower and smaller changes in gas port etc. have larger impacts.

To address the original topic, wouldn't a longer stroke would require an adjustment to the bolt hold open so the BCG doesn't get a running start before hitting it?

[mig1nc]

The overall gas drive (total area under the PV curve) can be made right by adjusting the gas port size, but that is not the same thing as optimal timing.

Gas port location is the most important factor controlling the timing.

Why do 14.5" carbines break more bolts than rifles?
TIMING


Agreed, but that doesn't mean a 16" carbine is the optimal configuration.

Isn't that one of the benefits of the mid-length system though, improved timing?

Could you not though use something like multi-vector gas tubes, fat boy gas tubes, or expansion chambers in the gas tube path to simulate the longer gas tube of the rifle-length system? Thus returning the timing to the original design? Using a 14.5" barrel with carbine length gas tube gets you the similar dwell time, then a modified gas path could get you back to rifle length unlock timing. Optimize with the correct gas port size.

[Clint]

Isn't that one of the benefits of the mid-length system though, improved timing?

Yes, exactly.

Could you not though use something like multi-vector gas tubes, fat boy gas tubes, or expansion chambers in the gas tube path to simulate the longer gas tube of the rifle-length system? Thus returning the timing to the original design? Using a 14.5" barrel with carbine length gas tube gets you the similar dwell time, then a modified gas path could get you back to rifle length unlock timing. Optimize with the correct gas port size.

Sort of.

Those things can be done, but add complexity and possible negatives, while changing the gas system length does this directly.

The dwell time (power time) should scale with other system times.

Keeping the same absolute power time as the rifle system while other variables change isn't the best approach.

Keeping the same RATIO of times is a better approach.

This was the driving factor behind the development of the EXT gas system for our OPTIMUM barrels.

[Clint]

My thinking is this- If carrier speeds are the same, timing is about the same.

Major factors that affect timing:

-Barrel length

-Gas system length

-Overall gas drive

-Moving mass

-Cam track geometry, specifically the straight part before unlocking starts


The critical timing event is when the bolt unlocks, relative to chamber pressure.

The later unlocking happens, the lower the pressure and bolt thrust forces are.

This lowers the friction and and stress on the lugs.

Carrier speed is what's left over after this all happens.

I'm also thinking that since the gas port is tapping into the barrel where the pressure is much higher which requires a smaller gas port to control flow, the flow hitting the expansion chamber hits the carrier with a harder pulse. Sort of like moving a ball with a slap instead of a push. This shorter, harder pulse would increase stress on the bolt lugs.

A shorter gas system starts pressurizing earlier, therefore the overall gas event is longer.

In order to have the same total area under the curve, the actual pressures need to be lower via the smaller port.

Longer and longer seem great, except this starts to happen while the chamber, case and bolt face are under higher pressures and not ready to be extracted.

This adds friction and stress that decrease consistency and reliability.

Does this make sense, or am I off my rocker?

Yes, of course you are. You have a serious condition known as EBRD.

[Rayrevolver]

Great discussion.

I am kicking myself for selling that Vltor "FOG" upper and never shooting it. That was the 10" barrel with a gas trap type system that required an expansion chamber.

With respect to the SF system, why wouldn't they have messed with the cam track? Does anyone know or have hands on one of these things?

[BufordTJustice]

Great discussion.

I am kicking myself for selling that Vltor "FOG" upper and never shooting it. That was the 10" barrel with a gas trap type system that required an expansion chamber.

With respect to the SF system, why wouldn't they have messed with the cam track? Does anyone know or have hands on one of these things?

Ray, if you analyze the pictures, the cam track is different. This is observable due to the extractor pin being further forward and extending past the front ledge of the carrier. This permits longer travel of the bolt.

Sent from my SPH-L720T using Tapatalk

[tom12.7]

I had wanted to step into some things in a little different way for possibilities with this system, then look into how we could change things in a more positive way for an AR derived DI system that works much better for many combinations overall, to reduce issues mostly. The timing for the initiation of unlocking to complete unlocking to the initiating of extraction to complete ejection, considering also possible residual barrel pressure blowing into the action when looking at different distinct barrel pressure curves (and cans), we can't handle those items well as we could in the base system. We can modify that base system to improve that, but where else are we lacking? One major one is within the magazine, the straight wall chute can not properly present a tapered case as cleanly or quickly as a proper case taper continuos curve magazine. When we expand this new concept, we can look into some other ones are more ambi friendly without some constraints, central plunger style over the one sided pivot.
Sounds great right? Some others thought so as well, but the base receiver enlarges in size, but not by a lot to do these things and some others. Unfortunately, sizing for these while being non AR compatible wasn't considered marketable. While I couldn't argue some of their points, it's really the market that drives demand.

[sinlessorrow]

Ray, if you analyze the pictures, the cam track is different. This is observable due to the extractor pin being further forward and extending past the front ledge of the carrier. This permits longer travel of the bolt.

Sent from my SPH-L720T using Tapatalk

There is a reason the KAC E3 bolt uses a spring retained extractor pin and why LMT added that extra bit to the front of their E-Carrier.

Unless surefire has a better way of retaining the extractor pin it will fall out. The carrier is what retains the pin in the AR bolt.