Hello will a single shot 12 gauge be an efficient hunter on whitetails?
 

hello im 13 and going hunting with my uncle... could i use my 12 ga singleshot ( h and r topper 158) smoothbore and loaded with 2 3/4 one ounce slugs, take down a deer accurately, and efficiently?

its got a bead sight...
what would be the max range i could shoot at, and could i attach a scope?

Well I'm no shotgun expert but, one shot is all you should ever need, so yes, it should work fine if you're using rifled slugs.
A scope would be a nice addition.
I wouldn't shoot past 50 yards. You'll have to play with it to find out what slugs it shoots best.

how would i mount a scope

Try shooting it. SOme of those H&R's are accurate , some aren't. I shot my first deer with a 20 ga. It ran a bit and tipped over.

what type of shells, and could i mount a scope?

I have been using Federal slugs for a long time. I use an old H&R 12 Gauge when I have to go into a shotgun area. I have a scope on mine, but the barrels had much thicker steel back in the 30's. I don't know about mounting a scope on a newer model because the barrels are really thin. The cheapest way to go is some type of rifle sights. They would make a big difference.

Sometimes I feel like I'm watching Candid Camera around here. You young guys can Google it and find out what I'm talking about.

This single shot, smooth bore will do fine for deer out to around 75-100 yards.

You'll be restricted to using a "rifled slug". These have been around for decades. They usually come in 5-packs. I suggest that you try two different ones to see if one of the two will group better than the other.

The issues with the rifled slugs are that generally they do not group all that well and theydefinitely slow down rapidly due to their poor aerodynamics. The arc is rather dramatic ... so pratctice at say 25-50-75-100 yards to get a feel for where you will need to aim at these distances.

As for a scope .... I cannot think of how a scope can be safely mounted to this H&R break action shot gun. It was not made for use with a scope. The barrel would have to be tapped near the breech and I cannot imagine how that could be safely done. And it would run probably $50-$75 anyway. Plus a mount set ... another $20 - $35.

If you clould find a better type of open sight.... say a front and rear set, that should suffice if your eyesight is OK,.

For your age I would go ahead and use your shotgun. Do not worry about a scope. I agree with buying a few different type of slugs and try them out to see which is the more accurate. At 13 this should be a good way to go as with it being only a single shot it will force you to take your time and think before shooting. Later on a scope and other items could be purchase when you get the money. Good luck and hope you continue to hunt and keep asking questions. Some forget what the OP asked and who he is.

This...just make sure you practice and hunt with the same shells...but above all make sure you shoot some BEFORE you go out so you have an ethical kill. Good luck! and post some pics when you get your deer!