Hello I am new to this forum and i Just bought my M&p 15t. When i shoot .223 ammo it will not always load my next round but when i shoot 5.56 i don't have a problem was wondering if anybody else has ran into this issue or can explain why its doing this.
I have to ask this question to get it out of the way. I don't mean anything by it, other than the question. (some people get their feathers ruffled too easy).
You said your M&P 15T is brand new. Before going to the range, did you field strip, clean, inspect, and lubricate your new M&P 15T?
Most firearms manufacturers apply a preservative oil to the firearms. They don't know how long a firearm will sit at a distributor/retail store until it gets sold, so they protect it. The preservative oil doesn't behave the same as an oil intended for lubrication, and can cause some sticking & binding.
If you haven't done so, field strip, clean, inspect & lube your 15T.
Now that's out of the way...
What lubricant are you using on your 15T? AR15's like to run "wet". Are you using enough lubrication?
Field strip your 15T. Check the bolt, upper receiver, charging handle, & charging handle channel for witness marks. Witness marks are points where the finish is worn off or scuffed due to rubbing. Those witness marks will tell you exactly where you need to make sure there is lube.
Is the gas tube clean & free of blocakge? 5.56 pressure > than .223 pressure. Your malfunction can be some sort of issue with the gas tube. I'd get some non-chlorinated brake cleaner, spray it down the gas tube from breech to muzzle to flush it out. Look through your handguard at the gas tube. Do you see any obvious kinks?
If that doesn't work, then maybe there is a leak at the gas block. Does it look like your gas block is properly installed? Does it have any play in it?
My 15-Sport is a tad over-gassed. This was to make sure that it would cycle any ammo I put through it. I switched the buffer to an H2 buffer to smooth it out.
Maybe your 15T is just right? In that case it's gassed to take 5.56. Does the buffer in your 15T have any markings on it? H? H2?
The other thing I need to ask is what brand & capacity magazines are you using? What are the details of the ammo you're shooting? Manufacturer, brass or steel case, etc...
That's about all I would do to diagnose the issue. If all of this still doesn't work, call S&W. The best part of S&W M&P 15 ownership is the limited lifetime service policy.