If mag-fed, you're usually restricted to the SAAMI max COALs, at best a little more irrespective of the rifle's freebore. In 223, the 69gn Sierra MK and its Lapua and other equivalents are designed for 2.26-inch COALs and magazine feed and that's what powder manufacturers' loads data show. N140 is a tad too bulky for 69s at this COAL. You can use it, but you're talking really compressed charges and a great deal of charge column decompression pressure, so neck tension has to be very high otherwise the bullet won't retain its original / proper seating position. When I shot lots of these bullets yonks ago in an AR-15 based (manual) rifle, I invariably loaded them with N135. Even with this faster burning powder and its lower charges, top loads saw some heavy charge compression and you could both hear and feel (through the press handle) the powder kernels crunch as the bullet went fully in. Not nice, and I never felt comfortable with this.
One tip for this kind of load combination is a powder funnel with a much longer drop tube than the half-inch stub on most standard models. Then use a very slow dribble pour, better still with the case/funnel angled to get a swirl pattern like water draining from a sink. I use the 5.25-inch tube Forster Universal funnel - very expensive but it works; useful in many 308 Win combinations too.
75gn bullets in the 223 are often more problematic and depend on individual model design as some are much longer than others. Long acetyl-tipped match bullets cannot usually be seated to magazine length as they end up far too deeply seated in the case. (Same applies but even more so to virtually all 80gn bullets, although some are just usable.) The 77gn Sierra MK is however, as with the 69gn model, designed for 2.26 COAL, but is again seated really, really deep with all that implies for powder charge compression. Same applies to its Berger, Lapua etc competitors. I'd advise against the [plastic] 'tipped' models such as the Sierra 69 and 77gn TMKs as they have to be seated deeper still due to the tip length, don't do a great deal for ballistic performance and cost another tenner a box. (Bling factor is high which is no doubt why they sell well!) Both N135 and N140 work fine in these loads, N540 too if you want some extra fps MV and don't worry about long barrel life.
Both 69 and 77gn bullets will perform fine in range-use at 500 yards, albeit with considerably greater windage than equivalent 308 Win loads and bullets.
On 308, it really depends on what you're shooting at with what kind of bullet. Is this paper or live quarry? What barrel length do you envisage? You can use some 180 and 200gn expanding lead-core, so soon to be banned, bullets in 308 at mag length, but I'll wager that several knowledgeable stalkers on this form will tell you to forget 200s and even 180s at 308 Win performance as it's not a big enough case / charge to produce high enough velocities. 165s are all sporting bullets; match models in this category are 168-175gn. There is only a single commonly found 180gn match design I can think of, and it's not one I'd recommend. (What am I saying? I've got 400 + 180gn Sierra MKs I'll never use and I'll sell them to you cheap - I'm serious, and they really are the finest things since sliced bread was introduced!
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For 500 yard range use on paper, the choice is huge, but the easiest option for 500 yards is to load a decent 155gn match bullet, again better without an acetyl tip if mag-fed. My favourite here is the 155.5gn Berger BT, but you need to search for anybody stocking bullets from this manufacturer then pay a lot (£95.95 / 100 is the UK distributor's RRP for this model). Sierras (two versions plus one tipped) are cheaper. For this distance, there is a lot of choice with old favourites such as the Sierra and Hornady 168gn models; Lapua 167gn plus various 175s from all the makers that are usually easy to get to shoot well. Some people claim to do well in this sort of shooting with the budget PPU match products - can't comment personally.
For 155-175s, Viht N140 does very well, as does the slightly slower burning Reload Swiss RS50 (and its 'hot' stablemate RS52). For 175s, I personally prefer N150, but if you're keen to stick to one powder for both cartridges, it has to be N140 and may have to be N135+N140 depending on bullet choice.
