Back to add a bit of hard data.
I'm a basic-science researcher, so I've got access to some nice, calibrated, laboratory scales. These were measured this AM on a Mettler B3002.
Above, I mentioned my old Western Arms Hi-Cap CQB Special. Dating from around 1998-99, this replica uses a "heavyweight" ABS-plastic slide. On the scale, it comes in at 144.36 grams. A 6061 slide of this configuration (full-sized 1911, 5" barrel) weighed in at 147.87 grams, and that's with a real Bo-Mar adjustable rear sight in-place. A 7075 slide came in at 162.44 grams, and that not only included the same Bo-Mar rear sight, but also a stainless-steel barrel bushing that weighed, by itself, 10.73 grams.
As you can see, like I said above, the slide alone makes very little difference, particularly if the replica already uses a "heavyweight" ABS-plastic slide.
Nevertheless, again as I've said above, there's other ways to add weight to the front of the pistol replicas, to try to shift the balance more forward, more akin to their real-steel counterparts.
My CQB is as-configured in factory-stock format. I've skirmished with it for more than three years, and it's held up beautifully - despite the plastic slide - with the exception of a few scratches here and there, kinda like my friend's Harrison 2011 that went through Costa's class a few weeks ago. :lol: This replica uses a lightweight aluminum metal outer cosmetic barrel threaded to a plastic cosmetic chamber section (both factory components - note here that depending on the replica, this may all be one piece - just like a real pistol's barrel is configured - but some replicas allow more customization/flexibility by offering it as a two-piece set of components), a setup coming in at 126.53 grams, complete with an aftermarket stainless steel two-piece full-length guide rod that I put in this replica, to enhance weight balance.
In my 7075 slide, for that particular replica, it's standard profile stainless barrel, coupled to a stainless chamber section, made for an assembly that's 149.36 grams. That's less than an ounce's worth of difference between it and the previous.
In the 6061 side, I chose to use a rather massive stainless-steel "bull" configuration outer barrel section as well as a stainless steel chamber section. This, coupled with a similar stainless two-piece guide rod assembly brings that set of internals to 180.22 grams. That's a big leap over the others, but that versus a real gun's barrel? again, no-contest.
Like I said above, we both agree that airsoft is but one of a number of good training tools out there. The end user really needs to decide for themselves what exactly it is that they need - what their training goals are - so that they can select the right tool for the job. For manipulations, the airsoft GBB replicas using a caseless 6mm BB leave a lot to be desired, and while a metal slide may add to a bit of tactile and cosmetic joy, they really do not offer considerable gains, otherwise, to the individual for whom the goal is Force-on-Force training.
Which reminds me.
Late last night, I made a rather rushed blanket-statement in terms of the suitability of these replicas for manipulations. Yes, I still maintain that things like malfunction drills cannot be properly executed with the caseless 6mm BB replicas and the slide/recoil-spring will not offer sufficient force to properly simulate that of a real pistol's, but for items like my Maruzen Remington 870 and 1100 replicas, their physical configuration as well as functionality really can offer very good manipulations practice. Sure, they don't weigh anywhere nearly as much as my real shotty, but again, it's having the right end-goals in-mind.
I really love the airsoft as a training tool - but it is what it is. Use it properly.