7 mm Texan Build

[dyotat100]

Here is the reamer for the 7mm. I just buy a reamer and have my tool grinder modify it. Put a 1 degree taper and a pin for a bushing on the end. Then make the bushing and glue it on with green loctite.

I got to test it today for numbers.

34" barrel dropped into 257 Texan. 100 gr slugs

2500 psi 990 fps
3000 psi 1050 fps
3500 psi 1100 fps

If I can get out to the desert I will shoot it. Not sure if I will get to though.

[mackeral5]

Over 200fpe on 2500psi, that is very impressive.  Out of curiosity, what pressure did the donor gun need to break 200 fpe as a .257?

[dyotat100]

Good question. My 257 condor with 32" barrel makes 200 fpe with 92 gr on 3000 psi.

When I make my 7mm I will use a valve like my condor has in the Texan but larger.

[mackeral5]

Good question. My 257 condor with 32" barrel makes 200 fpe with 92 gr on 3000 psi.

When I make my 7mm I will use a valve like my condor has in the Texan but larger.

The 7mm results are indeed impressive.  It would be awesome to see a somewhat apples to apples comparison between the 2 calibers showing pressure to fpe air usage, etc.  Sortof proving the set of hypotheses that led you all down the 7mm path. 

These results are indeed impressive and I cant wait to see how much more you wring out of it.  I also look forward to seeing 7mm used in a less radical application such as a tube gun with >28"  barrel.

[YEMX]

Those comparisons will be forthcoming- There will be at least 2 texans, one with a 34" barrel, and 28" barrel.  Kinetic45 is also building a bulldog and a Flex...  I think that should cover all the "typical" builds.

[mackeral5]

Those comparisons will be forthcoming- There will be at least 2 texans, one with a 34" barrel, and 28" barrel.  Kinetic45 is also building a bulldog and a Flex...  I think that should cover all the "typical" builds.

Looking forward to the BD and the Flex, 2 platforms I am very familiar with.  BTW, my last Flex is on its way to its new owner up in the Oregon area. 

[darkcharisma]

excellent work Doug!

i built a Rainstorm 257 and only 132fpe from a 24" barrel. i wonder if it could have been better to go with 32" barrels.

200fpe at 2500psi is exellent! it should be regulated for shot counts. 🤩

Doug, i asked you several moons ago about regulated chambers, i hope you can still make me one!

[mackeral5]

excellent work Doug!

i built a Rainstorm 257 and only 132fpe from a 24" barrel. i wonder if it could have been better to go with 32" barrels.

200fpe at 2500psi is exellent! it should be regulated for shot counts. 🤩

Doug, i asked you several moons ago about regulated chambers, i hope you can still make me one!

Here is some data from another tube gun but with a 28" barrel.   My QB78 .257 28" TJ's-- 155fpe at 3000psi, 169fpe at 3500psi  Pretty much full bore porting. It is much more tame/accurate around 140fpe though.....

[dyotat100]

28" barrel is done. Well done enough that I couldn't wait to shoot it thru the chrony.

28" barrel
100 gr
3000 psi
990 fps

It shot 987 fps to 992 fps. I'm not checking weight and they vary almost .5 gr.

Now I just have to glue the bushings with green loctite and then the carbon sleeve.

[rsterne]

Almost exactly what you would expect going from a 34" barrel to a 28" barrel.... Same velocity with a pressure of (34/28) x 2500 = 3036.... 

Bob

[Kinetic45^]

Note on my 34" barrel the CF sockets into the rear bushing and the front bushing goes OVER the CF tube so it's uninterrupted. The theory is it'll be stiffer over a longer length and have less harmonics. A PITA for Doug to build it this way for me though, thanks for going the extra mile!

On YEMX's 28" barrel the CF is a bit thicker and sockets into the front bushing ONLY and there's no CF between the bushings. I doubt that socketing in a short piece of CF between two bushings would increase stiffness much and would be too much work for the benefit (IMO).

Having the CF only on the front half of the barrel does help act as a resonance damper even if it has less effect on overall stiffness and of course CF adds to the 'cool' factor of a barrel. It should be noted that there's not much barrel sticking out of a Texan frame with a 28" though, it's just a stub, haha. [On the other hand, I've cursed the OAL of a long Texan plenty of times when moving and pushing crouched over through the South Texas coastal swampy almost impenetrable brush here where I live while hog hunting, haha That's one reason I love my Slayer so much, it's such a compact ergonomic package, even though I'm less a fan of bullpups overall.]

[darkcharisma]

Almost exactly what you would expect going from a 34" barrel to a 28" barrel.... Same velocity with a pressure of (34/28) x 2500 = 3036.... 

Bob

thanks for the formula, Bob
thanks Mike D for the comparison.

so (24/28)x3000=3600. i would need 3600psi to get to the same velocity with 24".
======
cool build, makes me wanna go larger than 257

[dyotat100]

You can't compare a tube gun to a AF gun

With 28" barrel in 257 I get 185 fpe.

[darkcharisma]

I know AF guns hold a better edge on FPE.

i have two barrels 257 and .308. i dont know which should be in my AF.

whats your highest fpe in 308?

[mackeral5]

You can't compare a tube gun to a AF gun

With 28" barrel in 257 I get 185 fpe.

I think this is the first time I've seen a decent comparison such as this.  It's probably been posted numerous times before, but thanks to this post I have a less fuzzy picture of the ceiling for tube guns vs. inline.  I know that my experience/skillset isn't as far along in development as others, so I think there's a little more meat on the bone for my little QB78.  I just need to find it.....  thx for sharing.

[Kinetic45^]

You can't compare a tube gun to a AF gun

With 28" barrel in 257 I get 185 fpe.

I think this is the first time I've seen a decent comparison such as this.  It's probably been posted numerous times before, but thanks to this post I have a less fuzzy picture of the ceiling for tube guns vs. inline.  I know that my experience/skillset isn't as far along in development as others, so I think there's a little more meat on the bone for my little QB78.  I just need to find it.....  thx for sharing.

A little off topic for the 7mm thread but I wondered if you'd tried the Cothran 7/8" Power Valve in your QB78? I gave a QB79 to a friend as a starter rifle and he upgraded it with that valve.... had to spin it and take about .002 off the OD of the valve with fine sandpaper to get it to fit the tube though.
https://cothranmachine.com/powerhouse-valve-for-7-8-250-transfer/

[mackeral5]

You can't compare a tube gun to a AF gun

With 28" barrel in 257 I get 185 fpe.

I think this is the first time I've seen a decent comparison such as this.  It's probably been posted numerous times before, but thanks to this post I have a less fuzzy picture of the ceiling for tube guns vs. inline.  I know that my experience/skillset isn't as far along in development as others, so I think there's a little more meat on the bone for my little QB78.  I just need to find it.....  thx for sharing.

A little off topic for the 7mm thread but I wondered if you'd tried the Cothran 7/8" Power Valve in your QB78? I gave a QB79 to a friend as a starter rifle and he upgraded it with that valve.... had to spin it and take about .002 off the OD of the valve with fine sandpaper to get it to fit the tube though.
https://cothranmachine.com/powerhouse-valve-for-7-8-250-transfer/

That's the valve I'm running....I've run in in many different configurations outside of what Don intended.  In stock form it produced the 150-170fpe numbers but of course with an extreme downward slope when not tethered.  The tube (also from Don C) is only 30" long, so 155ish cc of air.  Shooting tethered is the only option in this configuration.

I currently have the Cothran configured as a simplified balance valve and have an 8 shot tune with 53gr cast slugs that goes 915-935-910. 

I've invested a lot of time in testing the 7/8, .250 Cothran and currently have it in applications from 90 to 260fpe. 

[dyotat100]

I know AF guns hold a better edge on FPE.

i have two barrels 257 and .308. i dont know which should be in my AF.

whats your highest fpe in 308?

308 barrel won't work without a full custom valve. 257 is  best and if you want the most has to be custom valve

[Lani52]

I just break into 200 FPE with 75 grain 257-420 @ 1100 FPS 3600 psi, well a little less psi  than 3600 psi as the first shot was a err. 

My Haley 257 used to push that same bullet to 1235 FPS at 3300 psi, I had to turn it down, it had a 36” TJ.

Regards,

Roachcreek

[mackeral5]

I just break into 200 FPE with 75 grain 257-420 @ 1100 FPS 3600 psi, well a little less psi  than 3600 psi as the first shot was a err. 

My Haley 257 used to push that same bullet to 1235 FPS at 3300 psi, I had to turn it down, it had a 36” TJ.

Regards,

Roachcreek

Thank you for sharing this data.  That Haley was a heck of a bolt/tube gun.