Could a 303 bullet be used in a SLR rifle both British?

The answer is most certainly "NO".

Due to the perpetual stupidity of British military small-arms people, they decided to adopt a different way of describing calibres than everyone else.

The "303 British" calibre is in fact a BIGGER calibre than the "308 Winchester" which is the proper family name of the 7.62NATO cartridge used in the British "SLR" which was a variant of the Belgian FN Fusil Automatique Legere.

Small dimensions make a huge difference in guns.

303 British bullets are of a nominal diameter 0.3125 inches.

7.62nato (and 308 Winchester) bullets should be 0.308"

The difference would cause greatly increased PRESSURE if a 303 bullet was fired in a SLR rifle.

Moreover, most "303 British" bullets weighed around 174 grains, whereas the SLR's 7.62 bullet was 144 grains. This extra weight would also increase pressures.

As the SLR was not a particularly strong action, damage to the gun and injury to the person shooting it could easily result from any attempt to shoot 303 bullets from a 7.62 rifle.

Answer

303 bullet means- the bullet fits the barrel of a rifle, diameter of that barrel being .303 inch. this has no relation with SLR rifle

as someone that has used enfield number 4 mk2, an L1A1 SLR and an L42A1 quite a lot I'd like to add my 2 penneth worth here..

the .303 is also a rimmed cartridge wheres the 7.62 is not, so if you did managed to get it to feed the bolt won't be able to fully close because of the thickness of the rim catching on the edge of the chamber, I guess the most likely result would be either the weapon wont fire or if the weapon does fire you would have a breach explosion possibly to the anoyance to anyone on your right,

incidently as a side note, a number of old .303 rifles were converted over to 7.62mm and used by the British army for a number of years, designated as L42A1's before being replaced by the L96, maybe this could be the cause for confusion