I had a similar problem with a brand new XL7 30-06. First time at range sighting in I would put a cartridge in the magazine and on closing the bolt it would jump out of the mag. Had to feed by hand. I believed it to be a stiff mag spring. I took it home and loaded the mag and locked it in my safe for several days. Never had any more problems since. Hope this helps ya out.

 

I'm not sure of the problem you're having, but thought I'd mention something else on your bullet selections as you go along with that new rifle. I don't know the twist rate of the barrel on your rifle, but there is a lot of info/discussion out there on the .223 and weight/length of bullets it prefers for a given twist rate you might want to research. I haven't done a lot of experimentation with longer/heavier .223 bullets myself, but from my reading seems the caliber may be a bit more touchy(?) so to say when it comes to this issue than some others as far as ballistics/accuracy in a given barrel goes. Some .223 barrels are 1:7, 1:8, 1:9, and up to 1:14 or so in twist rates I believe with different performances as it were; regardless if in a bull barrel or not. Anyway, since you mentioned so many different bullets you want to use I thought I might mention it in case you were unaware? Good luck and good hunting to you.

 

Referencing previous threads, they indicate the .223 had a 1:9 twist rate. By "big barrel" are you referring to the heavy barrel aspect, or length? The weight of the barrel will have no impact on how well the gun will stabilize heavier/longer bullets.

Best bet is to try diff. weights and see what it likes, however based on my AR15 experience I'd say you probably won't stabilize anything much heavier than 64gr significantly better than you will 55-64. It's not always just a factor of weight and twist, but also bullet length itself. Some lighter longer bullets have issues stabilizing that shorter heavier ones don't always have.

With regards to using "68gr hunting ammo for coyotes and varmints", honestly I think it's overkill. Coyote are easy to kill, any good varmint bullet in the 40-55gr range will work extremely well. I've never had a coyote take more than a few steps after I hit them with 50gr vmax loads from my AR.

good reads..
http://www.huntingnet.com/forum/22-...2-centerfires/387666-223-barrel-length-barrel-twist-rates-bullet-size-type.html



"1-in-9 Inches
This is the beginning of the road for the shooter wanting to take advantage of the heavy bullet trend. The 1-in-9 is a great compromise twist rate—not too fast to cause problems with the 55-grain Bullets, but fast enough to stabilize all but the heaviest bullets under most circumstances. This twist will stabilize most traditional bullets up to 75-grains, and monolithics up to 70-grains—but they do so right at the edge of the envelope so not all rifles will do it. My personal 16-inch Rock River Arms carbine with a 1-in-9 twist does fine with ASYM’s Tactical Match Grade 77-grain OTM load, but has shown signs of instability with handloads using the 70-grain Barnes TSX—unless the bullet is pushed to maximum velocity. With longer barrels and the commensurate faster velocities, this twist can be more forgiving.

Read more: How to Pair Barrel Twist Rates with Bullets | Guns & Ammo"
​

 

Yikes!! Bullet "weight" and twist again....
Unless you ONLY ever use exactly one bullet shape, don't listen to bullet weight talk.

Twist coupled with velocity is what imparts RPM to stabilize a bullet. This has Jack shyte to do with weight, it matters to bullet length. The 77gr SMK is notably shorter than the 75gr Amax as an example.

So if you want an idea of having a prayer of stabilizing, look- up the lengths here, and input your specifics:
JBM - Calculations - Stability

Now as it relates to having too much RPM, here is your math: MV *720/TWIST= RPM.
As you can see the muzzle velocity plays a significant role in stability. Will the centripetal force overcome the centrifugal and make a Puff?? Well, I already asked that question of the mfgs for you, here:
F.Y.I. Varmint Bullet RPM Limits - Shooters Forum

Remember the actual twist and smoothness of your bore can move those numbers by a surprisingly large amount.

 

I have a bit of an update regarding the Lift Spring in the magazine; and it appears the majority of opinions on this manner seem accurate. That is; the lift spring put into the X7VH seem to be able to keep a Chevy Pickup from bottoming out. Mighty strong spring - and yep; I simply put in 4 Snap Cap .223 rounds and left it in the rifle for a week. Next time at the range it seemed to almost eliminate the problem by stretching out that new spring. I'm sure it will be even less of an issue in the future so Thanks - HairTrigger up in Northern Michigan. Your suggestion has been used and worked.