If every Ukrainian had a rifle and 300 rounds of ammunition, that might make the Russians rethink whatever invasion ideas they might have. The Ukrainian government should be building militia formations right now.
The most difficult part of making a decent rifle is the barrel. The machines to make even cheap barrels in an industrial scale run about $1 million but they can crank out a barrel every 5 min to 10 min or so. However, there are older ways to make barrels that are less costly. The trade off being that they take much longer to make a barrel.
If I were the Ukraine, I would be putting a few million of any EU or US aid money into building a couple of barrel factories. I’d also be working with existing machinists to crank out a few million gun parts. AKs are easy to make but are not as accurate as the M16 line. M-16 lowers can be made from casting 80% lowers and then milling out certain parts. I estimate that 1 machinist could mill out 2 lowers per day. Massive parallel 3d printing of AR lowers might work as an alternative.
Arm each citizen, train them in the care and use, train them in how to shoot accurately at 300 to 500 meters. Train 100 as snipers, then have that 100 train another 100 each. A nation of trained urban snipers would be DAMN difficult to overrun.