No it's a problem of some companies not doing the engineering when they put out a 7.62x39 upper and just machined out the face on a 5.56 bolt instead of putting the effort in and discovering that 7.62x39 weapons have more firing pin protrusion than 5.56 weapons.
It's an easy thing to fix, but...
Rifle length receiver extension and buffer/spring won't cause any cycling issues. But you will need to tap the detent spring hole, and trim down the detent spring a bit.
I was always fond of her using the 'Carter Special'
Twice the capacity and more powerful.
Though admittedly not nearly as pretty looking as your creation.
1 full pound for 20" barrels.
It's not the amount of the weight though, it's where it's located (out on that lever) so it seems heavier than it really is.
If all you do is bench shoot, or shoot prone, you won't notice the difference at all.
If you like to attend carbine classes, or...
If you do it at your house, or private range odds are they won't find out.
However, if you use a public range (or one where LEOs might be members), or for some reason are involved in a traffic stop and an LEO see the rifle.
He can run the serial number, they will know when you bought the...
By keeping it in boxes - makes it MUCH easier to count.
Keep the counts on a spreadsheet.
Reloads are kept in sandwich ziplock baggies (50 rounds each) or small 'snack' zip locks (20 rounds each) - with a slip of paper indicating the load and date the load was made. (I print the slips on...