Remington had a run of early 12 ga. 11-87 guns that the soldered stainless steel magazine tubes to receiver would come loose.
By the time the 1187 Trap model was introduced, this was no longer an issue. And it was an uncommon problem anyway.
It is a trap grade, now will this take rem chokes
The special trap chokes have the same outer dimensions and threads as Rem Chokes, and they are interchangeable.
The issue is the 1187 step rib trap barrel, as some noted above, is factory backbored. Instead of a nominal .727" bore, the overbore portion is .745". Just before the choke tubes, the bore steps back down again to .727". This leaves a built in constriction of .018", which is a mod choke.
So, if you were to install an ordinary Modified Rem Choke, with a .018" constriction, you have to ADD the built in barrel constriction to it, thus:
Barrel .018" plus Mod Rem Choke .018" = .036" Actual Total Constriction, which is a Full.
Barrel .018" plus Cylinder Rem Choke .000" = .018". You can never go below a Mod pattern, even with a spreader choke. I know, I tried doing so with mine for sporting clays. Found it best to simply get a 28" light contour field barrel for sporting clays, which BTW will fit the 1187 Trap forend just fine.
Barrel .018" plus Full Rem Choke .036" = .054", which is an Extra Full.
So what Remington did is they took the most useful chokes for trap, and remarked them as Trap Full, Trap Extra Full and Trap Super Full.
The Trap Full is .730", which is .003" above the nominal bore diameter of .727". Add that to the built in .018" and you get .021", which is between a Mod and a Light Mod. You would use this choke for singles and doubles.
The Trap Extra Full is .710", so it is .017" plus the barrel's .018" = .035", which is a Full. This is a good choke for advanced singles and doubles, and midrange.
The Trap Super Full is .691", so that is .036 plus the barrel's .018" = .054", which is an Extra Full or Light Turkey, depending on your shooting interests. This is a good choke for advanced midrange and handicap (long range).
Going further...
When lead shot enters a constriction, there is always some distortion to the shot, as lead is soft and it is being squeezed into a smaller space. Premium hard alloy shot has less distortion, while cheap soft lead has the most. Backboring allows the shot to open up in diameter a bit, then when it is squeezed in the choke, it is not squeezed nearly as much as a smaller bore and choke would do for the same effective amount of choking. This reduces flyers and ragged patterns.
I've found that this barrel will give nice patterns with cheap promo loads from Mao Mart, et al. This is nice because I baulk at paying extra for shells. Frankly and honestly I am not good enough to justify the added expense of premium shells, so this barrel suits me very well from a usage and budget standpoint.
Another thing Remington claims this barrel does, is make a denser center pattern by keeping more shot in the middle of the pattern. This makes a weaker outside pattern, but there are still enough pellets there to cover aiming errors. So even though in the math example above where it is shown the Trap Full tube for this barrel actually has the constriction of a Mod, if you patterned it you would find a more dense core, closer to a regular Full, with a secondary circle around it with less pellet density. Basically you are shifting pellets to the center of the pattern, yet are still getting a uniform outer pattern. This gives an equivalent pattern to a Full, even though the math shows it is a Modified. The harder the shot, the more this becomes apparent when patterned.
Can you use regular Rem Chokes in lieu of the specially marked trap chokes? Sure, but remember to always add .018" to their constriction.
BTW, note I said nominal bore diameter of .727". Remington 12ga bores actually range from .725" to .729" for field barrels. Their trap barrels have better tolerance, but still may vary. So while .727" is their advertised bore diameter, it could vary a bit.
Confused? If so, just go with this: Trap Full = short range, Trap Extra Full = midrange, Trap Super Full = long range.