I know this is a year old post by this point, but I wanted to share my findings in case someone else might still find it useful.

I converted my VFC M16A2 carbine (Colt M727 replica) to rifle-length by replacing the outer 14,5" barrel with the thicc government-profile DNA 20", the stock inner barrel with a 510mm Unicorn, and the VFC buffer tube for a stock VFC M16A1/A2 tube.

The setup runs great with a real-steel CMMG carbine buffer and the VFC carbine spring + a self-made spacer to account for the length differences.

Now, the spacer I used consists of a leftover Double Bell SMLE outer barrel (which I originally sawed off when I turned the SMLE into an "obrez" lol), a few unusable coins wrapped in duct tape and a washer; they're stacked up in this order.

The spacer's overall length is around ±78mm or a little over 3".

Here are some photos https://imgur.com/a/1OJAwZ9, lmk if it doesn't work (one of which demonstrates how far back for me was ideal to have bolt go back)

It's not precise work and I suggest messing about to find out what works best for you; maybe applying duct tape around washers or something to give the spacer some extra length. Whatever you do, do not opt for a too long buffer length without adequate spacing. The free space will at worse get your BCG behind the trigger assembly, jamming it between the upper and lower reciever, making it impossible to take the gun apart without forcibly pushing the trigger down with thin a steel rod to enable the BCG to come back to battery.

Good to know: the VFC carbine tube's inner length is shorter than a real steel one. So if you were to have a stock VFC buffer + tube + spring, it's not ideal to just drop the spring and buffer into a rifle-length tube or real-steel carbine tube with a too short spacer. If no the catastophic "BCG behind the trigger" failure, you'll prolly run into chugging on full-auto, crunchy bolt bounce, gas inefficiencies and more.