It is very tight. That's one reason the o-rings get shredded sometimes. juice on them helps some. The tapered drip tip or slap yo mama adapter really help. Got the slap yo mama today and it works great.
I used the BDV Lil Mama before the adapter came out and would use an old Ego battery to get a good force on the bottom of the carto to force it in. Now I've got a couple of the Slap Yo Mama adapters, they are magic! Don't need to lube the o-rings at all, could probably do it in my sleep. (that's what she said lol)