Took a few pictures today, hope that's good with Bullseye? If not just delete or I'll take it down.
To be fair/clear this isn't a proper toe to toe as I don't own a TAC 870. This is comparing my express supermag so a little different receiver and mag tube.
The main problem is, as pointed out, the mag tube length. I also think a sleeve could allow it to work. My Rem 18.5 lug fit right in, tho there was a gap at the lug when I tried the 26" vent rib, but you can see the lug is shorter than the other 2 when side to side. Couldn't figure the problem but I didn't thinker all that much (kids...) and to be honest I didn't even check if it does the same on my Rem, will have to double check.
Overall the Blueline is great for the money, definitely not perfect, but shot great, no hicups and 240 is just a ridiculous price. The follower works but it looks even thinner/cheaper that the stock Remington ones. The ribs on top of the receiver on mine are slightly off center and were machined a little on the left. I found the ejection a little underpowered, but that's not all that odd on Rem870s. It does feel a little heavier than my Rem and actually was even tho mine is more "kitted" (Blueline at a little over 8lbs and Rem just under in pictures). For those with a good eye, I did put an old wingmaster mag cap on the Blueline, hated the squarish stock one.
I do not regret the purchase one bit, but for someone that would like 100% interchangeability, this isn't it.