If you want to establish the maximum bullet length and size that you can sensibly use in your rifle, please take a look at
Refurbishing an ArgentineRB - Part11
(Restorer's forum) - which provides a detailed description of a method that can be applied to any bolt-action rifle. In particular, it illustrates that a too-short bullet causes insuperable problems.
Basically, I suspect (like others) that you have created your own problem. The Lee Enfield No.I MKIII was not designed as a varmint rifle, and there is no point in trying to turn it into one.
Follow the method outlined in the contribution mentioned above, and you will quickly have something that works as well as the rifle (and the "driver"!) will allow.
Patrick
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