The type of bullet is pretty much irrelevant.
The 3.5 tons is a British proof marking.
If your gun is a Smith & Wesson revolver, it's serial number most likely (but not necessarily) starts with either "V" or "SV". These were WW-II Lend-Lease guns provided to the British Army between 1941 and 1944. These guns are the
S&W Military & Police revolver and also known as "Victory" models. This gun in post-war years became the .38 Special Model 10 and the .38 S&W Model 11.
S&W .38 Caliber "Victory" Model (1941-1944)
The
.38 S&W Cartridge is different than the
.38 Special. It's a shorter, 'fatter' cartridge holding a bullet of nominally .360" diameter. In British service the cartridge was the .38/200 round (a .38 caliber, 200 grain FMJ). Standards U.S. load is a 146 grain bullet. The difference in cartridge sizes can be seen below.
.38 S&W (L) vs. .38 Special (R)
British .38 MkII 180gr
In the post-war years, many of these guns were "repatriated" to the U.S. But since the US military retained their .38 Special revolvers there was a civilian shortage. Enterprising folks on both sides of the Atlantic purchased war surplus .38 S&W revolvers, bored out the chambers in the cylinder to accept the longer (and slightly thinner) .38 Special cartridge. These were then sold all over the place from mail-order houses to hardware stores.
The bad news is that some of these "conversions" were sloppily done. It always behooves you to have your specimen checked out by a competent gunsmith.
Since the short/fat .38 S&W's chamber is slightly larger in diameter than the .38 Special with the re-bored chambers, fired brass will show a "bulge" in the lower portion, starting about .775" from the base of the cartridge. This is not dangerous, but makes reloading the empty case more challenging and tends to shorten their reloading life. It can often lead to brass splitting upon firing which will reduce velocity.
Since these guns are about 70 years old, they don't have the modern heat treating and metallurgy we take for granted. If you use .38 Special ammunition, be sure it is
Standard Velocity ammo.
Never use +P ammo.
If you're reloading for the gun and using .38/.357" bullets, use a hollow-base wadcutter or RNL (if you can find any) of moderately soft lead to improve accuracy. When fired, the "skirt" of the bullet around the hollow base will flare out some and get a better grip on the rifling. Reloading the .38 S&W can be fun too as it's a mild shooter. Finding .360" bullets may be something of a challenge however.