Arty-
Your own comfort is the only criteria that should be of concern in terms of heat; that and the mirage effect through the sights when using a scope, but this has never been a problem with me, even with high magnifications, on an AR variant. A 300-400 round session should include at least two to three cleaning/lube sessions if you're just plinking away, and you won't be doing any harm letting the piece cool down now and again between mag loadings... see to the bolt and carrier, punch the bore, lube all parts, etc. Any damage that could be done would be cosmetic, i.e. turning the gas tube blue, burning off/discoloring the finish on the barrel, but in either case, at this time of year and through a semi-auto piece, that's not likely to happen.
The most important consideration is to use a good quality name brand for your ammo needs, (NOT Wolf Brand!), and keep the moving parts clean and slippery. Do that, and you'll do fine. (And don't worry if you already have used any of the cheap blammo-ammo; just get rid of it and use something non-corrosive in a brass case.) You won't do any damage in terms of undoing any heat treatments, if that's what you're worried about...
Best of luck,