Does your flattop have an A2 front post & factory gas block or does it have a low profile gas block. If have a A2 post you have to choose if you want to remove it and replace it with a low profile gas block and a flip up front sight. Also you need to choose what type of rail you want. Quad rails aka cheese graters are out of favor these days. Most people are going to slimmer profile rails like keymod and M-lok which put rails sections on the top and sometimes bottom but not on the sides. You can then use the "locking" system to attach small pieces of rail where you need them. This makes for a lighter system that is more bare hand friendly.
This pic illustrates what an A2 gas block is.
Or do you have this?
For me its all about how much shooting you do with iron sights. If you shoot a lot with irons the A2 front post and a flip up rear or even a carry handle sight are a better setup. If you don't use irons and shoot mainly with a red dot or scope, which seems to be the trend these days, then BUIS flip up sights work better because 99% of the time they are out of the way.
If you are going to keep the A2 block, assuming you even have one LOL, look for carbine length 2 piece alum rail which can be had in different configurations like standard quad rail, keymod & Mlock. They can run you as low as $50 and as high as $200+.
You can also look at Magpuls MoE handguards which are drop in but made of plastic. They are very good and are very popular these days because they work and they are affordable right about $40.
Now if you want to remove the A2 post you will need to remove the gas block and replace it. To make the job easier you will want to pay someone to do it for you or if you do it yourself you'll need some AR specific tools. Receiver vice block and AR armorers wrench.
If you have the M&P Optic Ready I believe you will still need to replace the front gas block with a lower profile gas block in order to put a longer than carbine length FF rail. This is not as big a job as removing the A2 post but proper tools are still helpful.
The advantage here is that you can choose different lengths 7"-15" and freefloat the rail, so that it does not touch the barrel, which can cause changes in accuracy under certain conditions. These FF rails come in all sizes again in all sorts of configurations at all different price points but you can get quality keymod or m-lok rails for around $100. You will then need to add to that the cost of a low profile gas block and then a front BUIS, $25 and $40 respectively. This will be the more costly way to go but it does have some real advantages IMHO.
If you are going to build, work on or generally play with AR15 configuration getting the tools is a good idea. If you are going to only have 1 or 2 rifles and not really mess with them I suggest finding a local shop to do the change out.
