Squeeze Bore

The cone-bore principle was first patented in 1903 by a German designer Karl Puff. In the 1920s and 1930s another German engineer, Gerlich, conducted experiments with coned-bore barrels which resulted in an experimental 7 mm anti-tank rifle with muzzle velocity of 1,800 m/s.

Captured German 2.8 cm sPzB 41

Based on these works, in 1939–1940 Mauser-Werke AG developed a 28/20 mm anti-tank weapon initially designated Gerät 231 or MK.8202. In June–July 1940 an experimental batch of 94 (other sources say 30) pieces was given to the army for trials. The trials resulted in some modifications and in 1941 mass production of what became 2.8 cm schwere Panzerbüchse 41 started. One piece cost 4,520 Reichsmarks (for the sake of comparison, one 5 cm Pak 38 gun cost 10,600 Reichsmarks). The last gun was built in 1943; the main reason for the discontinuance was lack of tungsten for projectiles.

There was a gunsmith Arthur Langsford who lived in Broken Hill, New South Wales, Australia.  One of his inventions was the "Myra Extruder" (named after his wife). This is a .22 rimfire that has a forcing cone in just past the chamber and a different barrel either a .20 or .17 fitted. When an ordinary .22 caliber bullet is fired it is swaged down to the new caliber and the bullet exits with a slightly higher velocity, energy and better SD and BC. The .20 reportedly provided only a marginal gain over the .22 but the .17 gave impressive gains in velocity and penetration. The .17 took longer to perfect because a quicker rifling twist required.  Mr. Langsford also discovered that only 3% of the powder charge was needed to swag the bullet and that pressure wasn't significantly increased. (Note:  Langsford's has also been mis-spelled as Lansford and Langford.)

He would fit new barrels to your .22 or you could buy a Chinese made BRNO copy complete with synthetic stock directly from him. They cost around $600 last I heard.  Arthur Langsford has unfortunately died and these are no longer available. 

Myra Extruder on Brno 2E

Several years ago, we discussed this on the TC-List and there were at least two barrels, .22 to .17, reported to have been made for the Thompson Center Contender.  I don't know who the maker was, but using Mr. Langsford's method of long forcing cone should result in a relatively inexpensive barrel.  The question is whether or not one could use a relatively easily obtained .17 HM2 barrel or if one needs a more expensive custom rifled barrel with a faster twist.

It has been reported that Broken Gun Ranch (10739 126 Road, Spearville, Kansas 67876) is or was doing such conversions on Ruger 10/22s.  I don't know if that information is still good or not. Another stateside producer was Connecticut Precision Chambering who are reported to have produced barrels for the 10/22 and a few bolt guns and called their version a "Swager" barrel.  I have been told that Dave Van Horn also produced at least one such barrel for the Contender. 

- Langsford's Squeeze Bore Rimfires by Holt Bodinson, Guns Magazine January 2011
- Late pioneer beat the Yanks by Ross Williams, Weekly Times Now, December 2008