I have an older Bushmaster A2 upper with a 14.5" barrel with a pinned flashider.  I'd like to bring it up to modern standards and swap the upper for an A3 flattop (going to keep the the rest as is).  The existing upoer has no feed ramps while the barrel has the rifle style feed ramps.  My question therefore is how much hassle is to actually do something like that or would it be more cost-effective just to sell it and buy a new upper in the style I prefer.  If I do end up going with an jpper swap, what upper should I chose to match the fi ish on my mid-1990s Bushmaster lower (I presume I would need something without the M4 feed ramps to match the style of my older barrel)?

1) You will need to dig our the pin from the flash hider and the FH will be trashed. Mine was.
2) Either press or drive the out the pins holding the sight tower. Remove the front sight.
3) You will need a barrel nut tool to remove the barrel nut. Also need a vise and upper receiver clamshell holder.
4) grease the threads on the new receiver and assemble using a torque wrench to get the minimum 35lbs on the barrel nut.
5) reassemble the sight
6) put a new FH on and drill/pin/weld to barrel (need a welder, silver solder or take it to someone that can)

All that said, it depends on what tools you already have and if you want to go through the above steps to change to a flattop upper. Unless you already have all the tools (including the vise), it will be less expensive to just buy another upper in the configuration you want.

If it were me, I'd keep what you have and build up or buy another upper in the configuration you want.

Well it's easier if you plan to keep the same barrel nut. If you plan keep the barrel nut and are not going to free float it the process is fairly easy, knock out the FSB pins pull the FSB/gas tube forward. Loosen the barrel nut take off barrel, the repeat in reverse order on new upper. You will also need a new sight post to match up with the A3 upper. Busmaster sells them for like $7

Tools required

Armor's wrench
Vise
Upper vise block
Hammer
punch pins
rubber mallet (to knock the FSB forward if stuck)

If you are going to use the same barrel/handguards you don't need to remove the FSB pins.  Remove the gas tube roll pin from the FSB and remove the gas tube.  Remove barrel nut and remove the barrel from the A2 upper.

You will need to find a upper that DOES NOT HAVE M4 feed ramps.  You will need to call around to find one as they are not as common as they were a few years ago.

Install your barrel in to the new upper.  Tighten up the nut being sure to index the nut so the gas tube slides in.  Reinstall gas tube into the FSB and install gas tube roll pin.

This!

No need to bother with the flash hider or front sight base if you don't plan to free float the barrel.

Like others have said...If you're going to use the same handguard and front sight post...just do what was listed. Remove gas block pins, pull forward, undo barrel nut, pull it forward, remove entire barrel assembly. Re-mount everything to new upper and done.

If you want a different handguard...I'd just build a completely new upper (or buy a turn key...there's plenty of them on the market that are affordable). I'd also say that on this market...selling your current upper would also be a massive waste of $$...you're just not going to get much $$ out of it.

easy peasy but I will repeat.

knock out gas tube roll pin/ remove gas tube. put upper in vice remove barrel nut remove barrel from upper. Re grease the barrel extension with some aeroshell and the threads on the new upper. Re install barrel nut/delta ring. Re install gas tube and pin. Can be done in about 30 minutes with the right tools and taking your time.



butttt if it were me and parts being at all time lows. I would just "get both" and build a new upper how you want it.

These others are smarter than me... didn't think of pulling the gas tube pin. I guess I've been doing things the hard way

If you want to change to a free float hand guard you can always bob the front sight. I removed the sight , but you won't have to cause these nice gents provided a short cut.
Just cut with a hack saw, shape with a grinder and smooth with files. You would need to remove the sling swivel too if you have one. Used aluma black to finish.

Build another upper