Looking for an air pistol to dispatch rats for when the traps don't kill them.  I have narrowed it down to 13xx pump pistol but cannot decide on .177 or .22 for this.  Anyone have first hand experience with air pistol caliber and rats?

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Either caliber will be fine for that.  .177 pellets are less expensive but with the few shots taken to do that dispatch this cost difference will be negligible.  Also, for that job I'd suggest using wadcutter (flat nosed) pellets.  You are not worried about long range trajectory but rather the ability to kill quickly. Wadcutters, in my experience, seem to do this better than the hollow points or other fancy nose shapes.

If you are taking shots at some distance (significantly beyond 15 yards), then the domed pellets are the best choice.

What about noise, is one quieter than the other?  Does bore size make a difference in noise level with pellets?

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The .177 will be a little bit quieter, but not by a lot.  A good part of the noise will be determined by how many pumps you use.  For a rat at near contact distance three to four pumps should be sufficient.   It will be slightly quieter than driving an 8d nail into a 2 x 4 with a 12 oz hammer.

Yes both will do the job just fine, And at that distance only 3-4 pumps should do it for you.

I have shot rats before but never ones that get trapped and not killed.

Well, once upon a time I bought a refurbed 1377 from Randy at RL pretty cheap for this same reason.   It did the job okay, just had trouble hitting the vitals, they are very small, very fast, and the sights on the 1300 series aren't great.   Fast forward, I ended up making it into a carbine with a BNM receiver and shroud, stupid quiet, MROD mag fed, and scoped.   It works terrific but I ended up putting a ridiculous amount of money into it to make it perfect for that task.  5 pumps between shots gives me plenty of power.  So, a 1377 will do the job, just watch your wallet as money will tend to fly out when you discover how easy it is to mod those little pump-pistols.

I picked up a 1322 for $50.  I saw reference that the 13xx are the 10/22 of the pellet gun world.

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If you wind up going down that rabbit hole . . . .  Well, lets just say you should have started with a Marauder, SCBA bottle, and compressor.  You'd be in the same place financially.

My Dad has one, I'll go borrow his.

Much truth here.   Frankly, I'd just go for a PROD and a pump, you'll spend a bit more up front but it gives you what you really need.   I like my 1377 but I really did end up putting too much into it, basically started with a $50 dollar refurb and ended up with a $500 pump carbine.   It's a sickness.

The 1322 will blow through both sides of a steel can at 100.”  The .177 can only get through one side at half that distance.

Sure it can....with a longer barrel and an adjustable flat top valve.  And I doubt the 1322 in stock form outpenetrates the 177 gun at 8 feet with the same style pellet with the same number of pumps.

The 1377 can get through both sides with a steel BB.  The .177 pellet just lacks penetration ability against a steel can.

Move away from the pumpers and get a co2 1300 series and modify it for compressed air. If you'd like suggestions let me know, just know that it gets expensive quickly.

ETA: the co2 guns like the 2400kt are a lot more versatile. I converted one to a PCP...it got expensive but is silly quiet with a shroud and gets a ton of shots per fill on a small tank.

Thread resurrection due to my recent yard sale find.

Picked up a "phase one" 1377 at a yard sale. This is my first experience with one of these, seems like a fun little plinker to spend some time popping targets in the back yard.

I guess mine is the "first gen" of these and was made in '80-'81. After I googled the 1377 I see that the newer models have a bolt lever for cocking, while mine has a pull knob behind the bolt and a sleeve that covers the breech (chamber?) that I need to slide forward and twist.

I oiled the piston and after a couple of pumps/trigger pulls the piston swelled enough to actually hold pressure in the cylinder. Gun seems to function well, I just need to source some grips (stocks) for it. Maybe I'll just hollow out a potato and slide it up the grip frame?