1 in 8 wants heavy bullets and those Remington buckets are 55gr.
Try some 69-77gr stuff. Hornady do some steel cased 75gr training ammo, but for best results home loading is best.
2 die sets, bit with FL die and micrometer seating die.
The .223 set has less than a 100 rounds and the .308 has not been used.
.223 set £69
.308 set £76
Price includes postage.
124gr seems to be the best so far. About 250 rounds in and it shoots very well, 5 rounds within an inch at 50 and 10 round offhand groups within a 4" shoot n see. The main issue is only getting 2 hours a week to do any shooting.
Had mine about 10 years ago and the only place doing the work was Low Mill Ranges in Cumbria.
http://www.lowmillranges.co.uk
No idea if they still do that sort of work, but might be worth asking them.
There's a FB owner's group where some of the lads are using 12" Lantacs past 200 yards, they'll be the best to ask about ammo.
https://www.facebook.com/groups/Lantac.UK
The M4E1 is like the Mega Arms upper, where the upper fits inside the handguard, it's a decent set up and the only drawback being you can only fit Aero handguards.