Hi everyone I have a question regarding over all bullet lenght. For example my Hornady manuel states the max COL for a 303 british is 3.075. How low can you seat a bullet safely? I would like to seat some 172gr lead bullets at 2.797. Could this be an unsafe condition? Ideally I would seat just off the rifle lands but I felt I had to seat these bullets just past the last lube groove or does that matter? I have decided to get some opinions before test firing these bullets. Thanks in advance.

You can seat it as long as the chamber will allow.

The max OAL in the loading manuals is designed to ensure that it will chamber in all standard firearms of that chambering.

Given the 100+ year history of the .303, it wouldn't be surprising if it your particular gun had a chamber that would allow a longer OAL than the standard.

If you are shooting cast bullets - I'd personally try seating them to contact the rifling, and work from there.

As mentioned, the maximum cartridge overall length is intended to allow proper feeding through the magazine.
Your 172 grain cast may not need to be seated as long because it might be shorter than the bullet in the Hornday manual.
With this bullet I wouldn't worry about covering up the lubrication grooves, unless you are planning to carry them around a lot in your shirt pocket. I would worry more about keeping the base of this bullet inside the neck. If you protrude into the powder space there is a possiblity of gas cutting of the bullet.
I would suggest that you try seating with the base of the bullet at the base of the neck, and see how it shoots. You could seat them longer as well.

The Lee-Enfield has a throat long enough for the old 215 grain round-nosed bullet, seated to a COL of 3.075". As you noted, seating to the lands with a 172 grain bullet won't leave much bullet in the case.

Bye
Jack