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Discussion Starter · #1 ·
Why 11" Bbl Length for XCR SBR?

I know RA offers a 12" bbl for their SBRs....but I notice most folks cut down to 11" around here.

I have a couple of LWRC SBRs with 10.5" bbls. I am seriously contemplating getting a tax stamp for another XCR and getting 6.8 and 5.56 bbls and cutting them down (and obviously getting the 12" bbl gas dial to work with these bbls).

My question is: is 11" as short as you can go without affecting function? Would cutting the bbl to 10.5" put a Vortex flash hider too close to the gas block or something?

Also, I'm thinking of going with the HBAR profile on the 5.56 bbl so there's enough of a shoulder for the Vortex after the cut down and then having ADCO flute it to lighten the weight.....opinions?

Thanks,
Sean
 

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Re: Why 11" Bbl Length for XCR SBR?

I know RA offers a 12" bbl for their SBRs....but I notice most folks cut down to 11" around here.

I have a couple of LWRC SBRs with 10.5" bbls. I am seriously contemplating getting a tax stamp for another XCR and getting 6.8 and 5.56 bbls and cutting them down (and obviously getting the 12" bbl gas dial to work with these bbls).

My question is: is 11" as short as you can go without affecting function? Would cutting the bbl to 10.5" put a Vortex flash hider too close to the gas block or something?

Also, I'm thinking of going with the HBAR profile on the 5.56 bbl so there's enough of a shoulder for the Vortex after the cut down and then having ADCO flute it to lighten the weight.....opinions?

Thanks,
Sean
11" is about as short as you can go before the gun has cycling issues. Terra told me that Brandon needed a "piggy brake" to make a 10" work okay.

I'd like to get a factory 11" barrel if Robarm ever makes one. 12" works, but I'd rather not cut it down
 

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Discussion Starter · #3 ·
Re: Why 11" Bbl Length for XCR SBR?

I know RA offers a 12" bbl for their SBRs....but I notice most folks cut down to 11" around here.

I have a couple of LWRC SBRs with 10.5" bbls. I am seriously contemplating getting a tax stamp for another XCR and getting 6.8 and 5.56 bbls and cutting them down (and obviously getting the 12" bbl gas dial to work with these bbls).

My question is: is 11" as short as you can go without affecting function? Would cutting the bbl to 10.5" put a Vortex flash hider too close to the gas block or something?

Also, I'm thinking of going with the HBAR profile on the 5.56 bbl so there's enough of a shoulder for the Vortex after the cut down and then having ADCO flute it to lighten the weight.....opinions?

Thanks,
Sean
11" is about as short as you can go before the gun has cycling issues. Terra told me that Brandon needed a "piggy brake" to make a 10" work okay.

I'd like to get a factory 11" barrel if Robarm ever makes one. 12" works, but I'd rather not cut it down
Ya, I'd much rather just buy a factory one at 11", but I guess at this point, I'll go the other route (unless in the interim between tax stamp paperwork and actually getting a bbl there is a new 11" option from RA ;)).

Thanks for the info.

One last thing: do the 11" run flawlessly, or are 12" required to get 100% reliability? I don't want to do anything that jeopardizes full function.

Thanks in advance. You're a real asset to this and many other sites.

Take care,
Sean
 

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Re: Why 11" Bbl Length for XCR SBR?

11" factory light profile barrels are possible because Alex made one for me. He started with a 12" heavy profile, cut it down to 11", and turned it down to a light profile that way I had the shoulder for my flash hider or suppressor to but up against. VB is correct about the minimum length. You need at least 2" of barrel in front of the gas port for the XCR to operate properly (unless you dedicate a suppressor to it) and the gas port sits at 9". The PDW upper Alex is working on will have the gas port sitting closer to the chamber thus enabeling RA to make even shorter barrels work properly. The trick with the PDW is coming up with a shorter piston/op-rod that works well.

MadDog :rockon:
 

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Re: Why 11" Bbl Length for XCR SBR?

I can tell you that my 11" 6.8 doesn't need a new dial, it runs fine with the stock dial. The 5.56 barrel the same length needed a new dial (I have the dial but haven't tested it yet).

Speaking to Alex it sounds like the PDW is just about done but is not the focus right now (rightly so).

The XCR is a fantastic SBR, you wont regret cutting it down.
 

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Discussion Starter · #6 ·
Re: Why 11" Bbl Length for XCR SBR?

11" factory light profile barrels are possible because Alex made one for me. He started with a 12" heavy profile, cut it down to 11", and turned it down to a light profile that way I had the shoulder for my flash hider or suppressor to but up against. VB is correct about the minimum length. You need at least 2" of barrel in front of the gas port for the XCR to operate properly (unless you dedicate a suppressor to it) and the gas port sits at 9". The PDW upper Alex is working on will have the gas port sitting closer to the chamber thus enabeling RA to make even shorter barrels work properly. The trick with the PDW is coming up with a shorter piston/op-rod that works well.

MadDog :rockon:
That's really cool that Alex took care of you like that.....but I'd hate to inconvenience him. I'm sure he's got enough on his plate as it is with the XCR-M, new triggers, new stock, 6.8 1:11 twist bbls, 1:7 twist 5.56s, etc.



Thanks for the info on the 2" past the gas block....good to know. I'm looking more into doing this SBR now.....looking at forming an NFA trust through www.guntrustlawyer.com. Anyone have any experience using this guy? It's $600 for him to draw it up, which isn't chump change for me right now. I know I was able to do all the SBR paperwork myself for basically nothing by following the threads in the Class III forum over on ARFcom, but I'm not sure if you can do that while forming a trust.


Thanks again.

Sean
 

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Discussion Starter · #7 ·
Re: Why 11" Bbl Length for XCR SBR?

I can tell you that my 11" 6.8 doesn't need a new dial, it runs fine with the stock dial. The 5.56 barrel the same length needed a new dial (I have the dial but haven't tested it yet).

Speaking to Alex it sounds like the PDW is just about done but is not the focus right now (rightly so).

The XCR is a fantastic SBR, you wont regret cutting it down.
I don't doubt in the slightest that I wouldn't regret it. I LOVE my SBR 6.8/5.56 LWRCs.

I'm assuming I can just order the 12" dials with the conversion kit for 6.8, right? I know you're saying you don't need it....but if I'm ordering anyway, maybe I should just get the one that's supposed to work with an 11" bbl anyway?

Gotta try and sell off an AR to fund this though.

Sean
 

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Re: Why 11" Bbl Length for XCR SBR?

hmm, i think you should be able to have any size barrel you want, granted the person doing the job can appropriately adjust the gas system. EX: Tromix does custom work on S-12 shotguns, some customers have their barrels chopped down as far as 8"!!!, that requires them to break out the math to adjust and chop the gas system to the proper size.
 

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Re: Why 11" Bbl Length for XCR SBR?

Vultite: Not "every" gas system is created equal. I have a 7.5" barreled AR and guess where the gas port is? That's right, 2" behind the muzzle. I even saw someone cut an AR barrel down to 5" and it worked reliably, but guess where the gas port was? Yep, it sat right at 3" which is 2" behind the muzzle. I don't know much about shotgun gas systems but when you are talking about AR's and variants there of, the laws of physics are pretty hard to get around and still make the weapon 100% reliable.

Dog :tiphat:
 

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Re: Why 11" Bbl Length for XCR SBR?

I'm looking more into doing this SBR now.....looking at forming an NFA trust through www.guntrustlawyer.com. Anyone have any experience using this guy? It's 0 for him to draw it up, which isn't chump change for me right now. I know I was able to do all the SBR paperwork myself for basically nothing by following the threads in the Class III forum over on ARFcom, but I'm not sure if you can do that while forming a trust.


Thanks again.

Sean
I have heard good things about them but it is real pricey. I used quicken willmaker and have done 4 SBRs and I am waiting on 2 Cans, all on my trust. If all you are doing is putting NFA stuff in the trust I would buy willmaker. Save the other $550 for tax stamps. Most lawyers use basically the same thing anyways. They have special "lawyer" programs that fill in the blanks. :2cents:
 

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Re: Why 11" Bbl Length for XCR SBR?

Many of you are already aware of this fact, but the min. length is largely determined by how short one can make the operating system. Once the bullet has exited the barrel, internal pressures quickly equalize to atmospheric levels, thus no "juice" or energy to power the operating system.

Get too close to the end of the barrel, and the bullet has left before the "show", the gas bleed/piston movement/bolt operation has even begun. This is one of the reasons for muzzle devices like the Noveske KX3 and the "Krinkov" on SB AK's. They are there largely to sustain internal barrel pressures, and of course to provide additonal combustion of unburnt powder/reduce muzzle signature.

Alex has tackled the ultra Short SBR/PDW (think 9")design issue, and once he is able to make a determination of the optimal port size/twist rate.....Ummm, TASTY. Patience, but this is the closest thing that I have seen to a "nearly ready" product from RA. Once Alex settles on the final specs., it would seem to be relativley straight forward to place into production. Start saving for a new upper, because you are definitely going to want one.
 

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Discussion Starter · #12 ·
Re: Why 11" Bbl Length for XCR SBR?

Many of you are already aware of this fact, but the min. length is largely determined by how short one can make the operating system. Once the bullet has exited the barrel, internal pressures quickly equalize to atmospheric levels, thus no "juice" or energy to power the operating system.

Get too close to the end of the barrel, and the bullet has left before the "show", the gas bleed/piston movement/bolt operation has even begun. This is one of the reasons for muzzle devices like the Noveske KX3 and the "Krinkov" on SB AK's. They are there largely to sustain internal barrel pressures, and of course to provide additonal combustion of unburnt powder/reduce muzzle signature.

Alex has tackled the ultra Short SBR/PDW (think 9")design issue, and once he is able to make a determination of the optimal port size/twist rate.....Ummm, TASTY. Patience, but this is the closest thing that I have seen to a "nearly ready" product from RA. Once Alex settles on the final specs., it would seem to be relativley straight forward to place into production. Start saving for a new upper, because you are definitely going to want one.
I dont' really want anything under 10.5" or so anyway.....the ballistics get really marginal with 5.56 under 16"....under 10.5" is quite a bit worse still. Same with 6.8 (the two calibers of XCR I want). I'm not saying no one should get a PDW.....by all means, it's your money and if it fits what you want....go for it. I'm just not that excited for one, personally.

BTW, (Since I can't stay logged on on this site for some darn reason).....anyone know of anyone making a set of shorter BUIS like HK's diopters for rifles like POF, HK416, and the XCR that have a 1/2" or higher sight plane vs. an AR?.

I love my folder now, but the lack of a cheek riser is a problem to get the right sight picture for me. The easiest thing for me to do is get sights that are lower for my SBR project and get the "HK 416" LaRue mount for a T-1 Micro.

Sean
 

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Re: Why 11" Bbl Length for XCR SBR?

Many of you are already aware of this fact, but the min. length is largely determined by how short one can make the operating system. Once the bullet has exited the barrel, internal pressures quickly equalize to atmospheric levels, thus no "juice" or energy to power the operating system.

Get too close to the end of the barrel, and the bullet has left before the "show", the gas bleed/piston movement/bolt operation has even begun. This is one of the reasons for muzzle devices like the Noveske KX3 and the "Krinkov" on SB AK's. They are there largely to sustain internal barrel pressures, and of course to provide additonal combustion of unburnt powder/reduce muzzle signature.

Alex has tackled the ultra Short SBR/PDW (think 9")design issue, and once he is able to make a determination of the optimal port size/twist rate.....Ummm, TASTY. Patience, but this is the closest thing that I have seen to a "nearly ready" product from RA. Once Alex settles on the final specs., it would seem to be relativley straight forward to place into production. Start saving for a new upper, because you are definitely going to want one.

Welcome to the forum Schaferhund. You don't happen to work at Robarms, do you?
 

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Re: Why 11" Bbl Length for XCR SBR?

I personally don't have the money for NFA items, but if I were to do a SBR, I wouldn't want anything less then 12", and even then, thats only for a "room clearing" type issue, i wouldn't use anything below 16" for longer ranges. Preferably I'd like to get the XCR-M in a 18-20" barrel setup....
 

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Re: Why 11" Bbl Length for XCR SBR?

With regard to employment at RA, nope, just a friend of the company, and a true believer that the XCR is indeed a "better mousetrap". In fact, I was a somewhat reluctant convert...until I shot one.

With regard to the velocities/decreased energy associated with SBR .223's; take a long and very serious look at the 6x45 cartridge and the Alex/Cor-bon joint venture factory loadings. o313 and I spent an hour or so with a ballistics program, and 6x45 is an amazing little cartridge. New barrel ONLY. Same mags, brass, and bolt. +++ as opposed to 6.8 or 6.5 which have a host of issues associated with those logistical considerations(mags/brass/bolt). I will be building up an SPR/DMR designed to fire 105gr bullets with phenomenal downrange trajectory and energy retention due solely to the high BC's available in heavier 6mm/.243.

The 6x45 in an SBR does a great job of mitigating many of the negatives associated with SBR 5.56x45. Oh, did I mention you can load this starting with your 1F .223 brass, SAME Mags, and Barrel swap ONLY! Oh yeah, I did. I was all hot to trot on 6.5 and never really drank the 6.8 Kool-aid(though I wholly endorse the reasons for it's creation), but the 6x45 will be my new cartridge.

That said...boxers, briefs, or commando? To each their own, but everyone's observations have validity if they jibe with your goals and desires. I personally have an interest in an ultra-compact, gas piston, 6x45 PDW using 85gr bullets for EP work....so I am pretty stoked to see that Alex has something waiting in the wings.

Schafferhund
 

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Discussion Starter · #16 ·
Re: Why 11" Bbl Length for XCR SBR?

With regard to employment at RA, nope, just a friend of the company, and a true believer that the XCR is indeed a "better mousetrap". In fact, I was a somewhat reluctant convert...until I shot one.

With regard to the velocities/decreased energy associated with SBR .223's; take a long and very serious look at the 6x45 cartridge and the Alex/Cor-bon joint venture factory loadings. o313 and I spent an hour or so with a ballistics program, and 6x45 is an amazing little cartridge. New barrel ONLY. Same mags, brass, and bolt. +++ as opposed to 6.8 or 6.5 which have a host of issues associated with those logistical considerations(mags/brass/bolt). I will be building up an SPR/DMR designed to fire 105gr bullets with phenomenal downrange trajectory and energy retention due solely to the high BC's available in heavier 6mm/.243.

The 6x45 in an SBR does a great job of mitigating many of the negatives associated with SBR 5.56x45. Oh, did I mention you can load this starting with your 1F .223 brass, SAME Mags, and Barrel swap ONLY! Oh yeah, I did. I was all hot to trot on 6.5 and never really drank the 6.8 Kool-aid(though I wholly endorse the reasons for it's creation), but the 6x45 will be my new cartridge.

That said...boxers, briefs, or commando? To each their own, but everyone's observations have validity if they jibe with your goals and desires. I personally have an interest in an ultra-compact, gas piston, 6x45 PDW using 85gr bullets for EP work....so I am pretty stoked to see that Alex has something waiting in the wings.

Schafferhund
Agreed. And please don't take my comment the wrong way. PDW is cool as hell. If money were no object, I'd probably get one of those too.....but for me, the regular rifle with an 11" bbl and numerous length other bbls/calibers is more versatile.

Since I already have 6.8 and a decent stockpile of ammo and mags (that, and it's by far my favorite AR caliber to shoot....and I have a POF P-308), I'll just stick with it, though admittedly, the 6x45 round is definitely a step up from 5.56 and is more easily converted from standard 5.56.

Thanks for your input. Interesting/valid points.

Sean
 

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Re: Why 11" Bbl Length for XCR SBR?

With regard to employment at RA, nope, just a friend of the company, and a true believer that the XCR is indeed a "better mousetrap".

Just checking. Actually I was hoping we had another insider on the forum. :) Its really neat that you guys can drop in at Robarms and get the tour.

Take pictures next time! :)
 

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Discussion Starter · #18 ·
Re: Why 11" Bbl Length for XCR SBR?

Okay, with AG Holder's comments on a new, permanent AWB and Obama's website saying the same thing.....I think it's safe to say an AWB will become a reality at some point in time.

I'm contemplating holding off on buying a second XCR to SBR b/c I'm pretty sure ATF looks at making the SBR, not as modification of an existing firearm, but manufacture of a new one. Therefore, even if I have a 2nd XCR before a ban, if the ban passes before I get the paperwork done and receive my tax stamp, I assume it won't be legal to own b/c it's being "manufactured" into a SBR after the ban is in place, right?
 

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Re: Why 11" Bbl Length for XCR SBR?

They didn't touch NFA stuff last time, at least not to my knowledge. I purchased 2 cans during that time with no issues.
 

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Discussion Starter · #20 ·
Re: Why 11" Bbl Length for XCR SBR?

They didn't touch NFA stuff last time, at least not to my knowledge. I purchased 2 cans during that time with no issues.
I was told the same thing by Gordon Miller at HKspecialist.

Who knows what will happen this time around though.

I'm still on the fence. If I can find one, I'm tempted to get another. The whole 1:11 twist for the 6.8 thing being on hold is making me reconsider since I really wanted to get the 5.56 SBR'd and then do a 6.8 as well but with the better bbl spec.

I dunno....guess I've got time to think about it.

Sean
 
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