Q for those of you who build a mil-spec FSB T91 with .750 journal 14.5/16 barrel

[Prelude]

How accurate is your rifle?
What barrel did you use?
If a mid-length barrel is used, did you drill another gas port or used the mid-length one?
I have found the gas port I drilled on a pistol gas system barrel for my T91 build is almost on the same spot of a mid-length gas port.

Thank you!

[apex]

I originally planned to do this as well and did more research than I thought was necessary, but it wasn't enough.
So, the gas port on the T91 IS in the same spot as on a standard mid-length AR gas system, but it is drilled at a 30 degree angle like an AK. Whether this will correctly line up with a perfectly drilled .750 gas block is another issue since I never got this far.
The first problem now is how you are going to get the gas block to line up with the port and go past the block shoulder, as it sits about 1/2in further back than on an AR. Well, there's 3 solutions:
1. Get a shoulderless-barrel
This is the most expensive option, as I've only been able to find two companies that produce a shoulderless .750 .223/5.56 barrel (without going custom-made):
Ballistic Advantage "Hanson"-profile barrels: https://ballisticadvantage.com/14-5-inc ... arrel.html (14.5in), https://ballisticadvantage.com/16-inch- ... arrel.html (16in)
Bravo Company "Enhanced" barrel: https://www.bravocompanyusa.com/BCM-14- ... wf-std.htm
Both of these will run you over $200 for the barrel alone. If you want to do this on a budget this is not the best option.
2. Get a low-profile shoulder barrel and widen the barrel hole on the gas block
This was my second idea. I found this cheap beater Bear Creek .223 Wylde barrel on sale at Primary Arms and went for it (hey, it's on sale again!): https://www.bearcreekarsenal.com/bca-ar ... -1-8-twist
The gas block shoulder is very thin, and simply drilling out the gas block about 1/2in in to widen it to fit over the journal seemed like a good idea. I never went though with this though, but the parkerized Wolf A1 standard flash hider fits perfectly with the barrel's parkerized finish.
3. Turn down the gas block shoulder
This is the most dangerous option as this could bend your barrel and mess it up beyond repair very easily. Unless you are/can find a competant gunsmith who has experience in this I would advise not doing this.
4. Abandon the .750 barrel and just use the .708 gas block for the Wolf A1 barrel
This is the option I ultimately ended up going with since it required the least amount of work. Pulling off the A1 gas block can be a pain but with the proper tools (which I didn't have) it's a 5-second job. I just went ahead and got a 14.5in barrel, slapped the Wolf mil-spec block on, pinned and welded the mil-spec flash hider to the barrel so it's a 16in barrel (no ATF tax stamp or """handgun""" requirement), filed the weld down, and finally cold-blued the filed weld and bare metal spots so it's not really noticeable from a distance.

21684621654.JPG

I wish ye luck my friend.

[Prelude]

Thank you for sharing your experience! Much appreciated!
I have tried Anderson's 14.5 barrel with pistol gas system for my T91 build, it functions flawlessly after maybe 30 rounds break-in. It looks nice and everything but it shoots poorly. 3.5 inches group at 50 yards with factory ammo. Slightly better with hand-loads.
I am not sure if my drilling of gas port messed up the barrel, but without a bore scope, it is hard to tell.
Now I am planning your option 3, turn a mid-length barrel to remove its shoulder and test fit with my drilled mil-spec T91 FSB. If the gas port line up fine then I will test fire it. See how it shoots...
Wish me luck!
By the way, did you have to drill another gas port for your 14.5 wolf a1 barrel after the mil-spec FSB is installed?

[apex]

By the way, did you have to drill another gas port for your 14.5 wolf a1 barrel after the mil-spec FSB is installed?

Nope. Luckily the gas block I ordered had the port already drilled out, and it lined up perfectly with the existing hole on the Wolf barrel.

On the Anderson barrel, did you block off the previously-drilled pistol gas port and does the T91 gas block have a good seal on the barrel? Either might be causing gas to bleed off and could cause accuracy issues. Granted though, I wouldn't expect a sub-2in group at 100yrds out of an Anderson barrel.

[Prelude]

By the way, did you have to drill another gas port for your 14.5 wolf a1 barrel after the mil-spec FSB is installed?

Nope. Luckily the gas block I ordered had the port already drilled out, and it lined up perfectly with the existing hole on the Wolf barrel.

On the Anderson barrel, did you block off the previously-drilled pistol gas port and does the T91 gas block have a good seal on the barrel? Either might be causing gas to bleed off and could cause accuracy issues. Granted though, I wouldn't expect a sub-2in group at 100yrds out of an Anderson barrel.

I welded a regular gas block onto the shoulder of the barrel to cover the pistol gas port.
The gas block walked with just set screws, that's why I welded it.


The accuracy stays the same. (with set screws or welded gas block....)

That is good to know the gas port on wolf a1 14.5 barrel will line up with mil-spec gas block!

The mid-length barrel showed on the fist post is on the way to have its shoulder trimmed.
Will report back after the installation and test fire...

[niqstr]

hi. want to confirm what is the length on the t91 gas port. think sean@t91 said it was 8.5inch. is that your experience?

second i am getting 750 gas block to put on 16inch ba carbine. plan to cover it w upside down gas block. would there be interference w the piston rod cover?

will be drilling a new hole? is there a cobalt drill bit you can recommend? cuz all i see is triumph's wire drills at brownell?


also what type of drills do you recommend for qpq barrel? or do u recommend stainless steel?

[Prelude]

hi. want to confirm what is the length on the t91 gas port. think sean@t91 said it was 8.5inch. is that your experience?

second i am getting 750 gas block to put on 16inch ba carbine. plan to cover it w upside down gas block. would there be interference w the piston rod cover?

will be drilling a new hole? is there a cobalt drill bit you can recommend? cuz all i see is triumph's wire drills at brownell?


also what type of drills do you recommend for qpq barrel? or do u recommend stainless steel?

Is the T91 gas block gas port pre-drilled?
I think the best way to determine the exact location of the gas port is after you installed everything, including handguard. Make sure the gas block is straight, then pin the gas block, then drill the gas port.
At least it was how I did mine.
As to cover the existing carbine gas port, later I used countersink drill bit to open the hole a bit, wheel wire brushed and then weld it over. It is much cleaner.
I will recommend new drill bits, for all four barrels I drilled, they seemed to be fine if they are new and sharp. (One BCA, one Anderson, one Ballistic Advantage and a Green Mountain, yes, I made so many T91s.....)
Add more cutting oil and keep it cool by backing out the bit often. Let the bit bites the metal and back out. Take your time.
Place a cleaning rod inside the barrel so you don't over drill to other side of barrel and ruined the rifling.
Good Luck!

[niqstr]

hi and thank you 4 quick reply. mine is not a t91 build but will be using the piston rod& 750 gas block to create [hopefully] a working t91 style ar15 .

is the reason you had to do additional work to weld bec of gas leak? and what is the 'weld" mean per se... sorry not a gunsmith.

also did you measure the size of the t91 gas port? ba 16inch is around .065

you are lucky that you are betting on t91 gas piston system. at first wanted to get it 1 year ago but thought the proprietary system did not allow for plug & play. so did focus on the DI but luckily the more i read about the fouling of di, i finally went back to t91 & piston system.

i really like the unique design of piston system vs lwrc & lmt which looks very similar to hk/fn system?

[niqstr]

hi, looking at 16inch BA m4 profile. would the t91 750 gas block work on this ? esp
around the 9.5-10inch mark?

my concern is that around 11-12inch areas is bigger than 750 which t91 750 gb may not cross?

thx

n

[Prelude]

Hi,
I later realized that was easier and much cleaner to install the T91 handguard without a DI gas block just to cover the carbine gas port.
If you want to build a T91 with AR 15 barrel, I will suggest that you find a mid-length gas barrel to modify.
Here is how a gun smith turn down my barrel:



This will ensure your T91 gas block has good seal around the barrel. Most m4 style carbine barrel OD around 9-10 inch area is slightly smaller than .750. (Not bigger, but smaller)
My 0.02.

[niqstr]

hi thx 4 confirming that 9-10inch is smaller[side view looked smaller vs 11-14mark which is prob 750]

i am hoping that the pinning will secure it to the smaller diameter around the 9-10inch mark?

otherwise i might have to get 416r ba wylde barrel w midlenght.

how much did it cost you and what is the official process called for getting rid of the shoulder?

thx u for your time in replying back..

[Prelude]

My suggestion is to add a set screw under the T91 gas block if you want to go the M4 barrel route. (Carbine gas system)
I did this to my 9mm T91, same issue there, the barrel OD is too small.
That way you can semi secure the gas block before pinning it.





Otherwise it will not stay put.
I think it costs me almost $100 including shipping to get that barrel turn down. Labor is around $60.
It is costly, so later I tried to turn down the barrel myself, using router and wooden jig. They turn out well.

[niqstr]

wow yours look nice.

just bought the m4 barrel today.

so i will see how it goes. mail has been really slow. checking via tracking 4 t91 is almost annoying when u see "in transit" message 4 past week.

take care

[niqstr]

sorry but the t91 piston rod work[able to go through upper hole] in regular ie aero precision ar15 upper?

[Prelude]

sorry but the t91 piston rod work[able to go through upper hole] in regular ie aero precision ar15 upper?

the T91 piston rod is slightly larger than gas tube
need to enlarge that hole a bit

By the way, that upper was used to install the T91 Tactical 14.5" barrel.