if someone in jammu takes marks around 600+ in neet he won't be able to go to big colleges out of state due to 100% state quota?

The seats in Medical and Dental colleges all over India are divided as follows:

• 15% of the total seats are reserved for All India Quota. This means if there are 100 seats in any state, 15 of them are reserved for students from any other state of India. These seats are allotted based on their all India Rank after participation in All India Counselling
• 85% of all the seats are reserved for students from just that particular state (particularly in government medical college). This means no student from any other state can claim that seat.

So if you want a seat outside your own state, you will have to apply for the All India Counselling. You will register, fill in your college of choice and wait for the merit list. 600 is a good score to get into a lot of good medical colleges. It is not necessary to get admission in your own state, as seats are reserved for students who want to study in different states also.

This holds true for any college in the country. Even though you can apply for the state counselling of other states, but in that way you will only be eligible for the management seats. If you want a government seat in other states, it is best that you apply for the All India Counselling. You can apply through MCC's offical website which is given below:

BUT, since there is no reservation for AIQ in J&K, you will have to participate in your own state's counselling.