Big Guns is okay, but i recommend Small Guns for a few reasons. Testing and comparing damage, the XL70 is excellent and compares with the Avenger well. But with SG you get many more weapon options including the P90 (also great for bursting), the M79 (fun with grenade launchers), Pancor Jackhammer shotguns, and Gauss weapons if you can find them.
So going with your PvP reference, i will suggest this:
ST 1, PE 9, EN 10, CH 1, IN 2, AG 10, LK 8
Small Frame + your choice (Fast Shot or maybe Loner)
Small Frame is standard issue. Fast Shot does no AP bonus for bursts, but gives +5 damage per attack overall, not much. Loner helps you level.)
Don't worry about low ST. You will compensate with higher SG skill (beyond level 32).
Drugs: Buffout, Nuka Cola, Psycho, cigs (beer optional)
On drugs, he will have ST 4, PE 6, EN 10, CH 1, IN 2, AG 10, LK 8
Sight 38, DR 35 before armor, DT 5, Carry Weight 80.
Sight 38 allows you to see beyond the range of any SG weapon in (burst mode) except Gauss.
Tag: Small Guns, First Aid, Outdoorsman
Perks
Level 3: Quick Recovery (or More Critical for crit bursting)
Level 6: Toughness (or Even More Critical for crit bursting)
Level 9: Even Tougher
Level 12: Lifegiver
Level 15: Lifegiver+
Level 18: Bonus Rate of Fire
Level 21: Lifegiver++
Level 24: (your choice)
Notes
At level 1, read 10 Scout books and you'll have OD 130
OD 50 allows Gecko Skinning. OD 75 allows Snakeater. OD 100 allows Ranger and Cautious Nature. OD 150 allows Pathfinder and Explorer (Scout optional). All available at level 1 if you tag OD and read 10 scout books.
Level 1, get Boneyard Guard for free boost to SG. Put all Skill Points into gun skill until level 18, put next Skill Points into OD to 150 at level 19. Then SP goes to FA to 100. Plan to get Doc to 100 by level 32. Boost gun skill beyond 180 after that. (Gun skill 300 max means you do zero deterioration to small gun weapons).
If you go with the crit perks, i think it's more fun. If so, the last perk can be Toughness. Then you are a crit bursting tank with high HP and high DR.
Having this high OD allows you to return to fights much faster/easier when you experience combat failures. It's always an irritation to try to return to a fight still in progress, hear the action in TeamSpeak, but keep getting into forced encounters causing delays because your OD is 28 or whatever.
By level 44, he should have Small Guns 220, FA 100, Doc 100, and OD 150, but don't stop there - keep going.