My point was that if you have a large chamber and a close fitting bore the moderator will provide more suppression than a moderator wth a similar voume and a larger hole/bore.
It depends what you mean by "large chamber". I suspect that you are referring to the open chamber of a typical reflex design. This is what should be catching the majority of the muzzle blast, via a sort of internal muzzle brake. Preferably tightly toleranced and made from appropriate metal.
The baffle stack of such designs is mostly there to slow down the exit of the gases afterwards, once the bullet has left the far end of the mod. By which point it is of little relevance as to the fit between bullet and baffle, Elvis has left the building.
In your case a DPT 7mm baffle stack will be a tad quieter than the 30cal baffle stack you currently use, and the bullet will fit through it as the bore is approx 0.7mm smaller which is still 0.4mm larger than the .308 bullet.
Taking little account of the practical tolerancing of fitting some mod. onto some barrel thread, with whatever overhang of the baffle stack beyond that point. Might look like it works "on paper", probably will, but might not.
A small point, the majority of rifles have a right hand rifling twist. The majority of moderator threadings, as well as inserts, body parts etc. are also right handed. Thus the natural tendency of the combination is for the mod. or just bits of it, to work loose. Which they really can do. Catch it in time and tighten up again, fine. Otherwise it is a poor combination. From an engineering point of view.
But so many are used to RH twist barrels, how the wind lifts the bullet from the right, and drops it if from the left, as well as more esoteric stuff to do with Earth rotation, that it seems we are stuck with them. As are we with moderator parts and threads which go with the "tighty righty, loosie leftie" convention because otherwise some users might just ruin them, not realising the difference.
If you could cool you moderator it would provide more suppression too, maybe thats the next gen, finned mods ?
The time available to cool the gases is very small. Just as the properties of the metal from which the baffles are made is hugely different, in fact aluminium has one of the best combinations of specific heat capacity and thermal conductivity, but is pretty useless at withstanding hot gas erosion. Titanium is very poor on specific heat or thermal conductivity, but has its place, for some applications. Things like Inconel inserts are far superior, but probably not much relevant to us.
A practical mod. needs to be able to soak up the volume of gas, and heat, from the muzzle blast whilst releasing it back it back more slowly, and cooler. Meanwhile dissipating that heat through a good thermal pathway, the more mechanical barriers, threads, surface treatments etc. in-between the worse it gets. Hence fully welded, or 3D printed/sintered construction is good for that. Wrapping them up in insulating sleeves is not, a very bad idea, unless for some over-riding operational reason. Unless you simply want to shoot one, two or three shots, in a hunting scenario.
You can cool a moderator for a few shots by e.g. putting water, or some other fluid, maybe in gel form, inside, for a few shots. This might actually have some relevance for hunters.
Just as "first round pop" is easily explained. Products of deflagration of the powder are not fully combusted, they still have excess energy waiting to react with the oxygen in the atmosphere, either inside a mod. full of air, or outside a muzzle device, as in muzzle flash. Just as say a light coating of preservative oil inside might also do.