I have just had a look through various 18 pdr manuals (different gun I know) Shrapnel and H.E. shells seem to be propelled via the same amount of propellent.
Cartridge, Q.F., 18-PR., Shrapnel, Mark I. Charge consists of a 1 lb. 6 oz. 15 dr. bundle of cordite M.D. size 8.
Cartridge, Q.F., 18-PR., High Explosive Shell. Charge consists of 1-lb 6 oz. 15 dr. of cordite M.D. size 8.
The Shrapnel shell weighed a little more than the H.E., 18 lb. 8 oz. compared to 17 lb. 12 oz (later streamlined H.E. shells weighted 18 ld. 8 oz.)
It would appear that barrel wear from the two projectile would be comparable.
If you have different information then I'm happy to be proved wrong.
Quatermass