I see lots of guys buying STRIPPED lower AR receivers, but

 
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Things will come to a head once the People get their priorities right and stand up to the Tyrannical Government. It's coming soon to America.
 
Id point out that if your using the 223 cartridge any barrel length under 18" is costing you significant velocity,
velocity and energy you'll need on long range targets 20"-24" barrels allow you to put significantly more energy on target at longer ranges. longer heavier barrels generally reduce muzzle blast and reduce recoil and smooth function.
if all your shooting is at under 100 yards, a 16"-18" barrels adequate,
an 8" twist rate stabilizes most common bullet weights well,
yes LONGER & heavy fluted barrels
weigh considerably more,
and are not as compact or easy to use in confined areas,

but they tend to handle heat and remain accurate far better

looks damn expensive but might be well worth the price if you can't get to a decent gunsmith
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if you build a AR 15 with a 24" varmint weight barrel your rifle will weight close to 10 lbs but ar 15s like that,
used with the correct optics tend to be extremely accurate
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The 6.8 SPC is also loaded to a slightly higher pressure than the 6.5 Grendel (55,000psi vs 52,000psi).
the ideal 6.8 spc will have a 20"-24" barrel to take full advantage of the cartridges higher pressure and bullet designs
picture of 6.8 SPC vs 6.5 Grendel dimensions compared

6.5 Grendel vs 6.8 SPC Ballistics

Those differences in the external dimensions of the 6.5 Grendel and 6.8 SPC translate into some interesting differences in their ballistic performance.

This is illustrated in the table below comparing Hornady factory ammunition. The 6.5 Grendel load uses a 123gr ELD-M (.506 BC) while the 6.8 SPC load uses a 120gr SST (.400 BC).

Note that Hornady advertises a velocity of 2,580fps for this particular 6.5 Grendel load, but uses a 24″ barrel to do so. At the same time, they advertise a velocity of 2,460fps for their 6.8 SPC load with a 16″ barrel. In the interest of conducting as close to an “apples to apples” comparison as possible, I also included ballistic data for that same Hornady 6.5 Grendel load with a velocity of 2,369fps from a 16″ barrel.

All three loads used a 100 yard zero.

picture of 6.8 SPC vs 6.5 Grendel trajectory
As you can see, the 6.5 Grendel load using a 24″ barrel starts off with about a 12% edge in kinetic energy that increases with range (about 40% more at 500 yards). This is due to the fact that this particular load uses a heavier bullet with a higher ballistic coefficient and a faster muzzle velocity.

Additionally, that particular 6.5 Grendel load has a flatter trajectory at all ranges. Like with kinetic energy, this advantage grows as range increases: it has about 1″ less bullet drop at 200 yards, about 3″ less at 300 yards, about 7.5″ less at 400 yards, and almost 15″ less bullet drop at 500 yards.

However, things change quite a bit when the 6.5 Grendel is used in a shorter barreled rifle.

In that case, the 6.8 SPC actually starts off with about 5% more kinetic energy. The 6.5 Grendel is still using a heavier and more aerodynamic bullet though, so that slight advantage in kinetic energy decreases and eventually flips in favor of the 6.5 Grendel between 100 and 200 yards. At 500 yards, the 6.5 Grendel has about 15% more kinetic energy.

When fired from a 16″ barrel, the 6.5 Grendel also has almost exactly the same trajectory as the 6.8 SPC.

All that being said, the gap in performance between the cartridges is pretty minimal at ranges inside 200 yards.

The chart below compares how much a 10 mile per hour crosswind impacts those same 6.5 Grendel and 6.8 SPC loads out to 500 yards.

picture of 6.8 SPC vs 6.5 Grendel wind drift
Once again we see that the 6.5 Grendel outperforms the 6.8 SPC in terms of wind drift at longer range. This holds true for both barrel lengths, though the gap in performance is smaller when the 6.5 Grendel is fired from a shorter barrel.

However, the 6.8 SPC is still a very effective cartridge at short to moderate range.

The table below compares the recoil produced by very similar loads to the ones compared above for each cartridge when fired from identical rifles.

picture of 6.8 SPC vs 6.5 Grendel recoil
 



 
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just my input, from 50 years of owning AR15 rifles 16" barrels may look an feel cool, but the 20"-24" generally produced enough higher velocity
to make a VERY noticeable improvement in any cartridge velocity , in any caliber under .277, bullet diameter
and Id suggest a 18"-20" barrel is much more useful on most carbines than that handy 16" you see many guys select.
as to barrel weight, I vastly prefer the fluted stainless or chrome lined varmint weight barrel's as they generally hold onto better accuracy far more consistently when hot.
sure they weigh more, but an extra 2 lbs versus an inconsistent , wandering zero is a very small price to pay, in my opinion
and yeah a 7 :1 or 8:1 twist rate prefered on the 223.



 
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you don't gain much more velocity with longer barrel lengths with a 6.8 spc once the barrel length reaches 20" ,
but the heavier 20"-24" varmint weight barrels, if high quality, while heavy, do provide consistently very good and consistent accuracy
110 grain bullets seem to provide the sweet spot in bullet weight and performance.
yes a varmint weight barrel does increase the rifle weight up into the 7lb-8.5 lb range, but you get a very acceptable accuracy and lethal punch with low recoil, out past 500 yards. Id have to point out reality, in that the vast majority of cases games shot at ranges UNDER 300 yards.
and in my experience 6.8 spc ammos cheaper, far easier to locate, as is the brass, for reloading, and , in my experience,
the 6.8 spc II, mags and guns function more reliably.
 
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