Heat Treatment of Longbranch CNo7 Receivers

[purple]

There is a thread in the Marstar Forum concerning this, but there is no definitive indication of what heat treatment was applied to LB CNo 7 receivers incomparison to that used for their No4 receivers.

There is commentary about a different type of heat treatment being used for the CNo7 receivers which would render them unsafe to use if building a rifle for a high pressure round such as the .303, 7.62 or .223. The C No 7 receivers were built concurrently with the No4 rifles and it strikes me peculiar that a different type of heat treatment might have been used for them when production was only a fraction of No4 output at Longbranch. C No7 receiver production began in 1944 and continued through to 1953 anyway(I have one of these), so there is a possibility that different heat treatment may have been used for them over time.

It would be good to locate some actual manufacturing data/specs on these, otherwise this remains a topic of opinion and speculation. There are reports of .303 and 7.62 rifles being built on these. Does anyone have first hand knowledge/documentation on this topic? Any first-hand experiences with other than .22 LR chambered rifles built on specific year dated C No 7 receivers?

 

[tiriaq]

I have an old DCRA type 7.62 target rifle, built on a C No. 7 receiver, converted to a one piece stock, and fitted with a Canjar trigger. Apparently these conversions were done by a chap named Collins. The stock is an early Robertson. As a target rifle, it would have been shot a lot. I've fired it, no indication of any problems.
As far as I know, Lee Enfield receivers were hardened in the areas where the locking abutments are. Strength as well as wear resistance would be issues. The hardened areas can be seen on some receivers, and show up with exposure to chemicals on others.
But I cannot help with any technical data.

 

[Lee Enfield]

I've been told that no7 actions got no heat treatment, as 22 doesn't require it.

I have seen several no7 actions with 303 barrels and a few with 7,62 barrels installed.

I have never seen, nor heard of a factory DCRA 7.62 conversion of a no7 action.

I did talk to a fellow who had used no7 actions to build 7.62 dcra rifles, he claimed to have had them heat treated before conversion.

If you have a parkerized no7 action, you should be able to see the spot hardening on the left body side and right locking lug.

 

[purple]

I just spoke with an old friend who used to make up heavy barrel .223 Remington varmint rifles on both Longbranch and Savage No4 actions to see if he had ever done the same conversions on a C No7 receiver. He is an experienced 'smith and a no B.S. guy. I have examined some of his conversions and used to gopher shoot with him and some of his friends who were using them. They were not a thing of elegance and beauty with their one piece stocks and "truck axle" barrels, but they shot very well.

He advised that he had in fact done several .223 Rem conversions on the Longbranch C No7 receivers, and that these worked as well as the conversions done on No4 receivers with no failures or adverse effects. He confirmed what he had previously told me about not doing these conversions on the Brit made No4 receivers because he believed them to be weaker than the Longbranch and Savage actions.

The .223 Rem runs at 50-52,000 PSI pressures, so the fact that the C No7 receivers handled this round as well as the No4s tells me that heat treatment of the C No7 actions must have been on par with the No4s. Breech pressures for the .303 British are less than the .223 Rem.

 

[tiriaq]

Breech pressure is related to backthrust, but the area of the inside head of the cartridge case is also a factor related to backthrust. A small headed cartridge with very high operating pressure may produce less backthrust than a larger cartridge operating at lower pressure.
Rifles converted to 5.56/.223 had a reputation for shooting very well. Apparently reduced receiver flex reduced barrel vibration.
The rifle I described above was not a CAL 7.62 conversion, but rather a gunsmith job.