ciphery I suppose your asking me about the bullet jacket? The Speer and Prvi bullets are designed to expand more reliably in rifle caliber with a higher MV at point of impact at regular hunting ranges,the copper jacket is slightly thicker.
In other words at 100 yds. if the bullet isn't traveling at around 1800 fps. your not going to get proper expansion,it may expand somewhat or not at all and act more like a FMJ. Bullets that are specifically designed by the various bullet mfg. for the 7.62 x 39 Hornady,Speer,Barnes,Remington and Prvi all which are in the 123 to 125 gr. range have thinner bullet jackets and will expand more reliably with the MV the X39 generates.
This is why I stated that I like to keep my hunting distances short when I use 150 gr. bullets,mainly to assure I get good expansion. I haven't really had the opportunity to set up my chronograph at 100 meters,according to my load program I should have about a 100 fps. buffer using the 150 gr. bullets but from where I hunt it's not an issue anyways.
Here is a good example of what I'm talking about although the bullet construction is a bit different steel vs. copper so your mileage may very. This is a Wolf 154 gr. SP taken from a deer shot at about 80 yds. bullet struck the deer in the chest below the neck and logged in the left hind quarter.
As far as the old adage of cutting the bullet tip off to aid in expansion I agree most mil surplus FMJ ammo has an exposed base you never want to shoot jacketed bullets with lead exposed at both ends. There is a technique know as the 'dum-dum' bullet,it was a bullet used by the British in the 303 the bullets name comes form the arsenal in India where it was developed,basically they just took the FMJ bullet and turn it around to exposed the lead core base, This is not a new technique people that shoot subsonic rifle loads also turn the bullets base backwards to get more frontal impact from the slower bullet.
Here are two examples on is 30-06 mil surplus FMJ reversed and X39 subsonic load.