7x55mm Broadway Trust Co


#1

On page 68 of C. Yust’s “Cartridge Collector’s Notebook” there is a diagram of a very interesting-looking cartridge named the “7 x 55mm Broadway Trust Co.” The cartridge is a very unusual design in that the case has holes in it, so it resembles the 76 and 106mm Recoilless rifle cases. Only this cartridge is obviously much smaller.

There is a short papragrph on the page that reads:
“Development was sponsored by Imperial Chemical Industries in 1946. The project engineer was Charles Burney. The chamber void allows a constant high / low propulsion pressure.”

There is also a diagram of the workings of the weapon, which allows gas pressure to escape through these holes in the case. However, this escaping pressure does not appear to unlock the bolt of the weapon. Can anyone give my any more information on how this weapon / cartridge combination works, or any photos of the weapon or cartridges?

I googled “7 x 55mm Broadway Trust Co. Cartridge” and all it came back with that was relevant was the IAA cartridge / journal issues list in PDF form. It says there is an article on this round in issue 251/3. It calls it the “.270 Broadway Trust Company (BTC) Experimental”.


#2

As I recall that was Bob Leinendecker’s article and it is about as much as is known about the subject. Someone who has the DISC version of the IAA pubs. should be able to email that to you. Check with JohnS and JohnC- they have them.


#3

The 7mm BTC was unusual in virtually all respects. The case was perforated and the weapon had an air space around the chamber that was supposed to reduce pressure so that a straight spring retarded blow back action could be used.

The round was also unusual in that the bullets had a very high length to calibre ratio. There were several bullet types all about 45mm long. The lead cored types weighed 200 grains and the steel cored about 167 grns.

Cases were made in one piece and two piece types, the two pieces screwing together.

There was also an experimental 20mm version in 1944 that pre-dates the 7mm version which was active during 1946-48.

Regards
TonyE


#4

Burney was also the British designer who’s work on recoilless designs led to the Wombat AT recoilless rifle. I’ve sen the same drawings you have, falcon, and I have to wonder where the gasses went after they were vented from the cartridge; it seems to me that they’d have to go BACK, but I can hardly imagine that anyone would want to fire the bloody thing then :-)


#5

The drawing looks like the gases do not leave the chamber. I wonder what it means by “high/low propulsion pressure” ?


#6

This principle is used today in the 40mm grenade rounds, among others. It means that there is a small internal propellant container in which the propellant gas generates a high pressure immediately after ignition. However, instead of driving the projectile directly, this gas breaks through into a much larger volume where it can expand, thereby dropping the pressure. The projectile is then accelerated relatively gently, and the stresses on the gun are much reduced so it can be lighter. It only seems to have been successful in quite low-velocity applications, though.


#7

TAKE A LOOK AT THIS THREAD FOR MORE INFO ON THE HIGH/LOW PRESSURE SYSTEM.

General Ammunition Collector Discussion ~ GRENDEL SILENT 12GA


7mm British recoiless?