EDIT: Answer completely re-worked on June 5; some comments may no longer apply.
This comes down to two issues.
Issue One
What actually causes the detonation? The magic item says the beads detonate when exposed to magical fire or "when thrown". What is it about being thrown that allows them to detonate? Most likely the impact is the trigger that causes detonation once the beads are armed. The question is, what arms the beads?
Do they have to be primed somehow (like pulling the pin from a grenade), and if so how is that done (is simply detaching it from the necklace enough)? Do the beads somehow telepathically read the carrier's intent, so that they can distinguish being deliberately thrown from being accidentally dropped? Are they activated by reaching a certain velocity? Or are they always armed, and any impact above a certain threshold causes them to detonate?
If it's always armed or has to be primed before it's rammed down the barrel, then the blast from the black powder will almost certainly set it off (thanks Pulsehead for pointing out that it could be carefully rammed down the barrel without setting it off). If it's activated by the user's intent, then it can't be used with a firearm at all, since it won't detect that it's being thrown.
If, however, it's armed by reaching a certain velocity then triggered by impact, then you can probably use it in a firearm without any real risk of it detonating before it reaches a target after being fired.
As for once it's fired, if it strikes a target it should probably detonate immediately; the beads are intended to explode just from being thrown, so the impact of striking a target at firearm velocities should set it off instantly, rather than giving it time to penetrate. If it explodes from colliding with the target, there's not much the target can do to avoid the blast, so the target shouldn't receive a reflex save. Since the bead is being fired from a weapon, however, the attacker should have to make a typical attack with the firearm to actually strike the target.
Issue Two
Does the DM want magic and technology to be combined like this? While it can allow a lot of interesting creativity (such as firing fireball beads from a musket), it also probably opens the door to a lot of potentially game-breaking combos. Magic and technology together are likely to be a "the whole is greater than the sum of the parts" kind of thing. You might even take the route used in the Amethyst setting and say that proximity to magic actually prevents technology from working.
Creating custom items is always a matter of negotiation and judgment; there are no hard and fast rules. The books provide some guidelines, but are always quick to point out that they cannot be relied upon, and that the DM is always going to have to judge suggested custom items on a case-by-case basis.
The first guideline is to compare your desired item against the closest available comparable items, and adjust from there. Thus, to allow a ring, such as Devlin’s, to do what the quiver of Anariel does, you would start with the quiver of Anariel as your basis for the cost of doing so.
Furthermore, the passage on page 233 of Magic Item Compendium is really talking about adding new magical abilities onto an item that could otherwise have that ability. But ultimately, you can add anything anywhere as long as the DM allows it. The aforementioned guidelines suggest a 50% surcharge to add something to an unusual slot. So to put the quiver of Anariel effect on a ring should, by these guidelines, cost 50% more. On the other hand, the guidelines also suggest that something that does not use one of the usual magic item slots should cost double, so a magic quiver (which doesn’t prevent the use of other items) presumably costs double. So the guidelines would say that the ring should cost approximately 75% of what the corresponding quiver costs.
However, the problem here is that the quiver of Anariel is ludicrously overpriced, particularly at the regular arrows level. My games routinely ignore tracking mundane arrows in general, so they are literally charging 28,000 gp for a magic quiver that does what any quiver in my games does, by default.
On the other hand, magic arrows are a much bigger deal than mundane ones. Magic arrows partially stack with the magic properties on the bow shooting them, which is a big deal. While I’d say even Devlin’s ring is on the expensive side for mundane arrows, for magic arrows the cost definitely should be much higher.
So instead, I’m going to make a different suggestion: consider magic ammunition as if it were a 50-charge item, like wand. After all, enhancing 50 arrows costs the same as a sword. So if a 50-charge spell-effect item costs spell level × caster level × 750 gp, and a use-activated at-will item costs spell level × caster level × 2,000 gp, you’re looking at the use-activated version costing 2⅔× what the 50-charge version does. So if you apply a 2⅔× multiplier to the cost of 50 magic arrows (which is the same as the cost of a single magic sword), you have at least one reasonable idea for what this effect should cost.
The results are 5334 gp for +1, 21,334 gp for +2, 48,000 gp for +3, 85,334 gp for +4, and 133,334 gp for +5. Most likely you would ideally use equivalent levels of special weapon properties rather than straight enhancement bonuses, though.
Best Answer
Those magic items are legit enough… and there's more
Both the quiver of Anariel (Lone Drow Web column "Companions of the Hall") (28,000+ gp; 1 lb.) and the quiver of plenty (Dragon Compenidum Volume 1 139) (18,000 gp; 1 lb.) are overpriced compared to the quiver of lies (Book of Vile Darkness 116) (12,000 gp; 0 lbs.) if all the wearer needs are standard arrows and the wearer never plans to upgrade.
All three items have issues: the Anariel having appeared on Wizards of the Coast's sister site to promote material from Mirrorstone, its juvenile publishing imprint; the plenty first appearing in Dragon then in a Paizo-published but Wizards of the Coast approved text; and the lies never having been updated for the 3.5 revision. Nonetheless, as these are—so far as I'm aware—the only D&D 3.5e bottomless quivers, they're to what a D&D 3.5e DM is supposed to compare a new bottomless quiver to when a PC sets out to design a similar original magic item.
On the campaign value of endless arrows
I suspect having endless arrows from a quiver is usually less expensive than the weapon special ability endless ammunition because the weapon special ability endless ammunition means not needing a quiver at all. In some campaigns the difference between an archer needing to scour the enemy stronghold to recover only his +1 endless ammunition mighty (+5 Str bonus) composite longbow (16,900 gp; 3 lbs.) instead of both his +1 mighty (+5 Str bonus) composite bow (2,900 gp; 0 lbs.) and his quiver of lies is totally worth that extra 2,000 gp. That extra 2,000 gp also means only having to worry about protecting the magic bow rather than the magic bow and magic quiver—getting the magic bow made of the strongest material so as to avoid it being sundered is likely less expensive than getting both bow and quiver made of that same material… and, if the archer does not protect his quiver, some smart enemy's gonna come along and sunder that magic quiver precisely because the archer has not protected it!
(I ran a campaign where one of the PCs was a dedicated archer, and I explained well beforehand that as the PC advanced in levels and his reputation grew, he'd need to be more and more careful of enemies sundering his bow and quiver. The player appreciated my heads-up and had his PC protect the crap out of his bow and had his PC tote several mundane quivers. When enemies did finally start making sunder attempts against the PC's bow, the PC was ready—the bow was magically reinforced and the PC had a backup bow just in case. Ask if that's a thing, too, in your GM's campaign. If the GM will never sunder or scatter your stuff—and, seriously, some GM's just don't—, this is far less of a concern.)
Consider instead a haversack full of arrows
What an archer really needs to weigh these oodles-of-arrows options against, though, is a handy haversack (2,000 gp; 5 lbs.) and 60 gp for eight hundred arrows. This supply—plus however many arrows the GM lets the archer tote on his body—means even the most profligate of volley archers will have enough arrows to last at least a level or two. (An archer that uses an impressive and absurd 40 arrows per encounter still typically only needs 520 arrows per character level in D&D 3.5e, for example.) Seriously, for the price of a quiver of lies, an archer can buy five haversacks of arrows—that is, without needing to resupply, enough for seven levels of continuous volley fire as described above—and have cash remaining.
Seriously, by comparison, a haversack full of arrows—that are kept securely in their quivers or wrapped in blankets so as not to pierce accidentally the haversack—makes both a bottomless quiver and the weapon special ability endless ammunition an extravagance likely reserved for high-level characters.