@TheLBLman forgive my ignorance but what is HTL?
Pilchard, you're excused
Yes, HTL means
"Heavier Than Lead".
A common & popular version of HTL has been the "HEVI" brand and/or "HEVI-13".
HTL usually implies shot that is 8 to 40% "heavier" than lead, which equates to much greater penetration, and allows for the use of smaller pellets (denser patterns) while still achieving the penetration of much larger pellets.
For example, HTL in sizes #6 & #7 might have penetration and downrange performance more similar to a #5 standard lead pellet.
Technically speaking, one could call TSS just a more advanced version of HTL.
But it is
SO MUCH HEAVIER than lead, it is just referred to as TSS, for
Tungsten Super Shot.
Actually, what we're really talking about is the DENSITY of the pellet material, as a pound of lead, or a pound of HTL, a pound of TSS, or a pound of FEATHERS . . . . . a pound of each, is a pound.
It's just that a pound of TSS can about be held in a thimble, while a pound of lead needs two thimbles, while a pound of feathers may have the volume of 1,000 thimbles. Consequently, a "heavier" payload (such as 1 5/8 oz in a 20 ga, or 2 1/2 oz in a 12 ga) can be loaded with TSS whereas the hull space would not allow this much weighted volume of steel, or even lead.
Note that many steel waterfowl loads in 12 ga 3" magnum hulls may carry only 1 1/8 oz of steel pellets, simply because there isn't enough hull space to get much more in there.
To compare, Steel pellets are about 7.85 g/cc; Bismuth is about 9.7 g/cc;
Lead is about 11.1 g/cc.
The "HEVI" or HEVI-13 branded shot is about 12.0 g/cc (about 8% more dense than lead);
TSS is a whopping 18g/cc (62% heavier than lead!!!)
Depending on the purity of the source, all these various g/cc's may vary a bit.
Some ammo "brands" may "mix" various materials to produce something in between some of the pure materials. This may be done primarily because TSS is so relatively expensive, while steel & lead are relatively inexpensive.
But because TSS is
SO DENSE (and HARD), even #9 sized "dust" pellets can have enough energy at 60-plus yards to completely penetrate a turkey's head. If shooting TSS, one really doesn't need the more commonly used lead sizes of say #5 or #6, as the #9 TSS actually has comparable energy to a #5 lead, maybe even more penetration at 60-plus yards.
If you can afford it, and it's ok to use in your particular gun, TSS is simply a superior load. It is so good that a 20 ga with TSS can out-perform a 12 ga shooting lead.
Since one would typically fire only a very few rounds in a season of turkey hunting vs. a season of waterfowl hunting, TSS has become very popular with turkey hunters, but somewhat cost prohibitive for most waterfowlers.
Since TSS is "harder" than steel, it may not be safe to use large-sized TSS pellets (such as a #4 or a #2) in anything other than more open chokes (as is somewhat the case with steel pellets). Most people seem to think #9 TSS is ok for any choke.
To reduce costs, but still have loads superior to either steel or lead, some manufacturer are "mixing" TSS pellets with either steel, bismuth, or lead.