2 things you have to remember...Keep everything straight and square. Make sure the material you use to lift the receiver is doing so while keeping the bit centered on the spot you want to drill. A cheap solution is plastic cutting boards since they are fairly easily cut and shaped. If anything is tilted/canted/crooked your hole will be too, which could cause the rivet to not want to sit flush like it should. If you don't already have a small machinist square handy you could probably pick one up at Harbor Freight cheap so you could square up the receiver to the bit. Also either make a centering mark where you want the hole to be or use a split point drill bit so it doesn't want to "walk" off center when you try drilling it.
Don't know how much slop your DP has but keeping everything as close to the head as possible will usually give the best results. By that I mean you should set everything up to where the bit only has to move a small amount before it contacts the metal and starts to cut. This gives the most rigid setup on a DP. To check the slop you extend the drill quill and check it along the way by grabbing it and trying to make it move side-to-side and front-to-back, usually the further from all the way up the drill chuck gets the worse the slop becomes which is why you only want to move it the shortest distance possible. Hope this helps.