There's a new 7mm / .284, 18" twist barrel available for airguns

TJ's now has the mandrel for making barrels in 7mm for those who want to try this caliber.

I had the mandrel made by Pacific Tool (took them over 4 months) and it cost $265. I did this because I believe there is an unfilled niche between .257 and .308 in bullet weight / FPE and that the frontal area of the .257 is not enough but the .308 bullets with enough frontal area have too poor a BC in the weight most airguns can push.

This new barrel has 6R rifling for better engagement with the rifling and should have reduced fouling due to the land profile.

The R style rifling was first used in Russian AK47s so the sides of the lands are ramped / sloped instead of being 'straight'.
Later Obermeryer used the concept to create the match barrels with 5 lands where the odd number and shape of the lands deformed the bullet less on engraving for more accuracy.
This type rifling cuts into the bullet less and also eliminates the 'pocket' or corner in the barrel where the land and groove join which usually accumulates leading and where brushes don't reach well in cleaning. [Note: also that this rifling hammer forges easier]

here are two links that explain and picture this 5R rifling
https://faxonfirearms.com/blog/what-and-why-5r-rifling/
http://www.obermeyerbarrels.com/faq.html

Since "5R" is copyrighted I could not use it, so I have done an even number rifling more like the original Soviet rifling.

And why an 18" twist?
TJ's and other barrel manufacturers already sell barrels with 14' twists (and even quicker like 11' and even 7 inch twists) BUT that fast a twist is for fairly long and heavy bullets.... TOO long and heavy for most airguns to utilize well . Yes an AirForce or other high power airgun could push a 120gr bullet to decent velocities (which to me for slugs is 925 FPS and above) and utilize that twist but most tube and valve guns would take a lot of extensive/expensive modifications to be up to that range of FPS.
Looking at the potential of most tube guns they are roughly capable of around 150/180 FPE (larger caliber, bigger transfer porting and valve). So the bullet weight has to drop to fit in the formula to get the 925+ FPS to give a decent trajectory and a better wind deflection profile.
Note that a LOT of my thinking has been derived from Rstern's articles ALONG WITH a lot of practical experience hunting where I saw the importance of 'bigger caliber is better' in lethality. Did a bunch of ballistics testing down through the years too!

Let me first emphasize that I am writing about SLUGS, not pellets here.
I've got everything from .22 and .25 Marauders to rifles in other calibers like souped up WAR FLEXes (.22, .25, .30 and .357), Slayer and every Texan caliber so I have seen that although the .25 family {.250 slugs of weights of 26gr to 55 gr; .257 slugs of a range of 60 to 90gr} does fairly well, most guns are having a harder time getting decent velocities with the heavier weights. And the performance of the lighter weights in each caliber for range and accuracy lose something.
The same model guns in .30 caliber family shoot the lighter weight .300's of the range of 42 to 62gr and in .308 bullets that range from 95 to 155.
There is overlap here between the two calibers but I believe there is a sweet spot in bullet weight between the .25 and .30 where 7mm / .284 bullets of a weight range of 80 to 100gr would fit in well. And bullets of that weight stabilize better at an 18" twist and not well at all at 14' twist which I think is whyt earlier experimenters did not get very good accuracy.

Most rebarreled tube and bottle guns should be able to get sufficient FPS (and hence FPE) to do well with this weight range. And these bullet weight range projected BC is better than the .30 family lightweight bullets of the same weights, not much but a bit of an edge enough.

I'm making this mandrel at TJ's available to all airgunners in the community who want to try this interesting caliber. Just think, only a few years ago there were no .250 caliber airguns, only .177 and .22...
Then we got .257, the .300/.308, the .357 and .457 as factory guns (and recently .40 and .50!).
Maybe if you of the airgun community find there is value in the 7mm/.284 someday there might be factory guns in this caliber too.
And I have a project in the works to re-barrel a Benjamen Bulldog to 7mm. Won't that be interesting?

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Heres what diameters of TJ's barrels are available:
.688" OD (11/16)
.562" OD (9/16")
.551" OD (14mm)
.500" )D (1/2")

Todd at TJ's told me that they may be able to make 7mm in 5/8" OD but would have to get a special order from their supplier as they don't generally stock the blanks in that OD for this caliber.

Their email is: [email protected]
And their phone number is: 859-635-5560
There is no web page/site

I've ordered two 34" barrels with an OD of .562 which will get sleeved with CF from Clearwater Composites with .563x.691 tube for a barrel swap into my .257 Texan (one goes to me and one goes to Dyotat100 for his rifle)
And I've also ordered a .500, 24" barrel to go in my FLEX+ (or SuperFLEX, haha) where it will join the barrel collection of .250, .308 and .357... all slug barrels with slug twists, no pellets shot here)

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