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OK a little advice needed. I reload under the tutelage of a bench rest shooter with 40+ yes experience. However he has no experience reloading for magazine fed rifles. When we looked at seating depth the bullets ended up getting pushed right in to the case whenever we reached the depth that would fit in the mag. After much finicking we got them all done. Shooting today and I had a stoppage. When I cleared it the bullet had again dropped inside the case. I checked others in the mag and 2 of those had done the same (under recoil I guess). Its as if the necks are not tight enough. Could it be a problem with the die? Brass was PPU which is cheap I admit but is it that awful? Bullets were 80g sierra MKs. Any AR reloaders had similar issues? My reloading teacher is clueless on this subject. All advice appreciated.

You are using the wrong bullet to seat to magazine length in an AR. The bullet is too long. You will be seating the bullet so far into the case that there is a possibility that the neck of the case is trying to grip the ogive of the bullet instead of the parallel portion of the bullet.

You need a new teacher by the sound of things!. The simplest and safest way to start is to seat the bullets to SAAMI spec. this would be an overall length of between 2.125" and 2.26". Rounds loaded to those dimensions should fit every .223 on the planet.

Your bullets should not be dropping into the cases, are the cases trimmed to length and full length resized?, what bullets are you using and are you crimping them?. Can you post a photo of one of the loaded rounds?.

Edit.

Just re-read and you're using 80gr SMK's, these are not suitable for mag fed AR platforms, the Sierra info says the following, "However, it must be seated to an OAL that requires single loading in the M16/AR-15 series"

80 grain bullets will not seat mag length in a .223, it's designed for slow fire single fed Highpower shooting. Thats maybe why you got them cheap, it's a good bullet but not many people shoot them these days as Highpower is not as popular as it was at Bisley.

If you want hand loading advice for shooting an AR why not ask the people who shoot them regularly? I do not doubt that your mentor has lots of bench rest experience but you should not have even attempted to seat the 80 grain bullets mag length. Quite frankly you are lucky you did not have a catastrophic failure that could have resulted in injury.


read this

Will also echo what Phaedra has said, you may need a new reloading teacher.

The 80 grain SMK is certainly not a bargain bucket bullet with some of the UK outlets selling them at GBP180/500, probably because they are hard to source right now. They point very well and are a superb long range .223 bullet, certainly better than the Berger offerings in my opinion. Downside is they have got to be hand fed in an AR platform.

For precision shooting at 1000 yards they are hard to beat with the .223.

Ok so it looks like I need to source some appropriate bullets.
Any recommended brands/weights? What have you guys found to be successful and reasonably priced?

69 or 77 grain SMKs

Thank you again John. I will get some ordered. On the plus side the reloads for my .308 worked out beautifully so the afternoon was not a total write off...

Before you rush off what's the twist rate of your barrel? for 69gr and above a 1 in 7 or 1 in 8 would be best, they will also handle lighter bullets like 55gr.