I have a rifle that I really like. Was built over the years for 3 gun, it's beefy but a really soft shooter and runs perfectly. Problem is, it's a heavy beast. It's got a jp 18" bull barrel, Magpul UBR, lancer lower, 15" FF tube and trijicon 1-4.

I was thinking of lightening it up since I don't shoot much 3 gun any more, basically turn it into a more tactical or in close rifle. The problem is everything is fully custom painted and I like it that way. I couldn't change anything without ruining that besides the barrel.

Looks like the barrel weighs about 35 ounces. for comparison, a 14.5" pencil weighs 16. seems like a good way to shave almost 2 pounds off a 12 pound rifle.

Is this worth doing? Or would I be better served just building a lighter weight upper for my SBR lower and leave this one alone?  The sbr has a 7.5" barrel with incoming can. Nothing really done to lighten it

Not really wanting to do a whole new gun at the moment, just something a little more handy.

Why don't you go ahead and swap to a lighter barrel, run it and see how you like it that if it is not what you envisioned, use that barrel as a start point for the new build and put meaty back on?

Even long pencils handle like a wand.  

If you like the balance and handling of the heavy barrel, be prepared to have to lighten the butt end when you swap to a lighter barrel.

I’d suggest a new light upper and doing what you need to do to your SBR lower to achieve good balance and handling.

JPK

If I have a gun configured in a way that works well for its intended purpose, I generally leave it alone.

I buy/build guns for a purpose.  So, if I'm in your shoes, I'm not re-configuring a 3-gun rifle...esp if I like the way it shoots and there is some possibility I might use it again. If you chose to reconfigure it anyway, you will essentially be building a new rifle given that you will have to replace the back end to even out a lighter barrel....which then screws up your paint job.

Personally, I'd just build up a purpose built upper to meet your needs and/or a new AR build.  Parts are so cheap now-a-days, that it is generally not cost prohibitive to do so.

I too have an SPR barreled rifle that started out for 3 gun but just felt too heavy for any other kind of use.  I eventually built a 16” medium barrel weight which in the end is only about 15 oz lighter overall vs the 18” and is used now for 3gun where as the SPR has a 10x scope on it.

Sounds like what you need is a nice 11.5 or 12.5 barreled upper to put on the SBR lower.  This would bridge your current gap almost perfectly.

My go to carbine started with a BCM 16" standard profile barrel and 15" handguard. I saved about a pound and a halfish by going to a 14.5 ELW with pinned A2X and a 13" handguard. The difference to me was absolutely stunning. Then again it went from fairly light to really light.

I imagine going from really heavy to not-quite-as-heavy will be less of a jaw dropper but itll still make quite a difference, imho.

ETA even if you decide to build up your SBR, if you dont need the heavy barrel for 3gunning and the gun becomes a range toy I would still slap a lighter barrel in there just for comfort.

Sounds like you need to leave it alone and just build a new AR that is lighter!  

Besides that barrel changes are easy and cheap relatively speaking.  A 16" lighter weight profile barrel would be a few ounces lighter and ditching the UBR in favor of something lighter would also be a significant area where you could lose weight.